Improve CI documentation test

[pmeier]

I think we can make two improvements:

1. Disable the latex build. The argument for building the documentation within a PR is to catch errors early-on. For that task we do not need to build the html and the latex version: one is sufficient. Since the latex build needs more resources we should disable it in favor of the html build.
2. Currently warnings raised by the build are ignored. We should build with the -W flag to turn them into errors.

Cc @jbueltemeier

[pmeier]

Actually, we should make this even stricter with the -n flag. This way we would be warned if we have broken references. Right now, there are a lot of them:

/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_loss.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.content_loss:2: WARNING: py:class reference target not found: pystiche_papers.gatys_ecker_bethge_2016._loss.FeatureReconstructionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_data.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.images:2: WARNING: py:class reference target not found: pystiche.data.collections.download.DownloadableImageCollection
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_utils.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.multi_layer_encoder:2: WARNING: py:class reference target not found: pystiche.enc.multi_layer_encoder.MultiLayerEncoder
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_utils.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.optimizer:2: WARNING: py:class reference target not found: torch.optim.lbfgs.LBFGS
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_loss.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.PerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_utils.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.preprocessor:2: WARNING: py:class reference target not found: torch.nn.modules.module.Module
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_utils.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.postprocessor:2: WARNING: py:class reference target not found: torch.nn.modules.module.Module
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_ecker_bethge_2016/_loss.py:docstring of pystiche_papers.gatys_ecker_bethge_2016.style_loss:2: WARNING: py:class reference target not found: pystiche_papers.gatys_ecker_bethge_2016._loss.StyleLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_loss.py:docstring of pystiche_papers.gatys_et_al_2017.content_loss:2: WARNING: py:class reference target not found: pystiche.ops.comparison.FeatureReconstructionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_loss.py:docstring of pystiche_papers.gatys_et_al_2017.guided_perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.GuidedPerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_loss.py:docstring of pystiche_papers.gatys_et_al_2017.guided_style_loss:2: WARNING: py:class reference target not found: pystiche.ops.container.MultiRegionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_data.py:docstring of pystiche_papers.gatys_et_al_2017.images:2: WARNING: py:class reference target not found: pystiche.data.collections.download.DownloadableImageCollection
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_pyramid.py:docstring of pystiche_papers.gatys_et_al_2017.image_pyramid:2: WARNING: py:class reference target not found: pystiche.pyramid.pyramid.ImagePyramid
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_utils.py:docstring of pystiche_papers.gatys_et_al_2017.multi_layer_encoder:2: WARNING: py:class reference target not found: pystiche.enc.multi_layer_encoder.MultiLayerEncoder
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_utils.py:docstring of pystiche_papers.gatys_et_al_2017.optimizer:2: WARNING: py:class reference target not found: torch.optim.lbfgs.LBFGS
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_loss.py:docstring of pystiche_papers.gatys_et_al_2017.perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.PerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_utils.py:docstring of pystiche_papers.gatys_et_al_2017.preprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePreprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_utils.py:docstring of pystiche_papers.gatys_et_al_2017.postprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePostprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/gatys_et_al_2017/_loss.py:docstring of pystiche_papers.gatys_et_al_2017.style_loss:2: WARNING: py:class reference target not found: pystiche_papers.gatys_et_al_2017._loss.StyleLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.batch_sampler:2: WARNING: py:class reference target not found: pystiche_papers.data.utils.FiniteCycleBatchSampler
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_loss.py:docstring of pystiche_papers.johnson_alahi_li_2016.content_loss:2: WARNING: py:class reference target not found: pystiche.ops.comparison.FeatureReconstructionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.content_transform:2: WARNING: py:class reference target not found: pystiche.image.transforms.core.ComposedTransform
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.dataset:2: WARNING: py:class reference target not found: pystiche.data.datasets.ImageFolderDataset
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.images:2: WARNING: py:class reference target not found: pystiche.data.collections.download.DownloadableImageCollection
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.image_loader:2: WARNING: py:class reference target not found: torch.utils.data.dataloader.DataLoader
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_utils.py:docstring of pystiche_papers.johnson_alahi_li_2016.multi_layer_encoder:2: WARNING: py:class reference target not found: pystiche.enc.models.vgg.VGGMultiLayerEncoder
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_utils.py:docstring of pystiche_papers.johnson_alahi_li_2016.optimizer:2: WARNING: py:class reference target not found: torch.optim.adam.Adam
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_loss.py:docstring of pystiche_papers.johnson_alahi_li_2016.perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.PerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_utils.py:docstring of pystiche_papers.johnson_alahi_li_2016.preprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePreprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_utils.py:docstring of pystiche_papers.johnson_alahi_li_2016.postprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePostprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_loss.py:docstring of pystiche_papers.johnson_alahi_li_2016.regularization:2: WARNING: py:class reference target not found: pystiche_papers.johnson_alahi_li_2016._loss.TotalVariationOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_loss.py:docstring of pystiche_papers.johnson_alahi_li_2016.style_loss:2: WARNING: py:class reference target not found: pystiche.ops.container.MultiLayerEncodingOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_data.py:docstring of pystiche_papers.johnson_alahi_li_2016.style_transform:2: WARNING: py:class reference target not found: pystiche.image.transforms.resize.Resize
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_nst.py:docstring of pystiche_papers.johnson_alahi_li_2016.training:2: WARNING: py:class reference target not found: torch.nn.modules.module.Module
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/johnson_alahi_li_2016/_modules.py:docstring of pystiche_papers.johnson_alahi_li_2016.transformer:52: WARNING: py:class reference target not found: pystiche_papers.johnson_alahi_li_2016._modules.Transformer
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_loss.py:docstring of pystiche_papers.li_wand_2016.content_loss:2: WARNING: py:class reference target not found: pystiche_papers.li_wand_2016._loss.FeatureReconstructionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_data.py:docstring of pystiche_papers.li_wand_2016.images:2: WARNING: py:class reference target not found: pystiche.data.collections.download.DownloadableImageCollection
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_pyramid.py:docstring of pystiche_papers.li_wand_2016.image_pyramid:2: WARNING: py:class reference target not found: pystiche.pyramid.pyramid.OctaveImagePyramid
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_utils.py:docstring of pystiche_papers.li_wand_2016.multi_layer_encoder:2: WARNING: py:class reference target not found: pystiche.enc.multi_layer_encoder.MultiLayerEncoder
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_utils.py:docstring of pystiche_papers.li_wand_2016.optimizer:2: WARNING: py:class reference target not found: torch.optim.lbfgs.LBFGS
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_loss.py:docstring of pystiche_papers.li_wand_2016.perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.PerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_utils.py:docstring of pystiche_papers.li_wand_2016.preprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePreprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_utils.py:docstring of pystiche_papers.li_wand_2016.postprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePostprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_loss.py:docstring of pystiche_papers.li_wand_2016.regularization:2: WARNING: py:class reference target not found: pystiche_papers.li_wand_2016._loss.TotalVariationOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/li_wand_2016/_loss.py:docstring of pystiche_papers.li_wand_2016.style_loss:2: WARNING: py:class reference target not found: pystiche.ops.container.MultiLayerEncodingOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.batch_sampler:2: WARNING: py:class reference target not found: pystiche_papers.data.utils.FiniteCycleBatchSampler
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_loss.py:docstring of pystiche_papers.ulyanov_et_al_2016.content_loss:2: WARNING: py:class reference target not found: pystiche_papers.ulyanov_et_al_2016._loss.FeatureReconstructionOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.content_transform:2: WARNING: py:class reference target not found: pystiche.image.transforms.core.ComposedTransform
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.dataset:2: WARNING: py:class reference target not found: pystiche.data.datasets.ImageFolderDataset
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.images:2: WARNING: py:class reference target not found: pystiche.data.collections.download.DownloadableImageCollection
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.image_loader:2: WARNING: py:class reference target not found: torch.utils.data.dataloader.DataLoader
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_utils.py:docstring of pystiche_papers.ulyanov_et_al_2016.multi_layer_encoder:2: WARNING: py:class reference target not found: pystiche.enc.models.vgg.VGGMultiLayerEncoder
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_utils.py:docstring of pystiche_papers.ulyanov_et_al_2016.optimizer:2: WARNING: py:class reference target not found: torch.optim.adam.Adam
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_loss.py:docstring of pystiche_papers.ulyanov_et_al_2016.perceptual_loss:2: WARNING: py:class reference target not found: pystiche.loss.perceptual.PerceptualLoss
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_utils.py:docstring of pystiche_papers.ulyanov_et_al_2016.preprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePreprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_utils.py:docstring of pystiche_papers.ulyanov_et_al_2016.postprocessor:2: WARNING: py:class reference target not found: pystiche.image.transforms.processing.CaffePostprocessing
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_loss.py:docstring of pystiche_papers.ulyanov_et_al_2016.style_loss:2: WARNING: py:class reference target not found: pystiche.ops.container.MultiLayerEncodingOperator
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_data.py:docstring of pystiche_papers.ulyanov_et_al_2016.style_transform:2: WARNING: py:class reference target not found: pystiche.image.transforms.resize.Resize
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_nst.py:docstring of pystiche_papers.ulyanov_et_al_2016.training:2: WARNING: py:class reference target not found: torch.nn.modules.module.Module
/home/user/pystiche_papers/.tox/docs-html/lib/python3.6/site-packages/pystiche_papers/ulyanov_et_al_2016/_modules.py:docstring of pystiche_papers.ulyanov_et_al_2016.transformer:2: WARNING: py:class reference target not found: pystiche_papers.ulyanov_et_al_2016._modules.Transformer

[pmeier]

Two more options:

1. doctest: While not yet included, we might add code examples to the docstring. With this sphinx extension we can detect changing output.
2. pydocstyle: Equivalent of flake8 for docstrings.

[pmeier]

One more option:

• pycodestyle has the option to set maximum allowed doc line length.