Problem with custom mapper

[mjack3]

Hello, I am following this in order to learn how to use a custom mapper.

So at the moment I just copy the example in the documentation

def custom_mapper(dataset_dict):
    dataset_dict = copy.deepcopy(dataset_dict)  # it will be modified by code below
    # can use other ways to read image
    image = utils.read_image(dataset_dict["file_name"], format="BGR")
    # See "Data Augmentation" tutorial for details usage
    auginput = T.AugInput(image)
    transform = T.Resize((800, 800))(auginput)
    image = torch.from_numpy(auginput.image.transpose(2, 0, 1))
    annos = [
        utils.transform_instance_annotations(annotation, [transform], image.shape[1:])
        for annotation in dataset_dict.pop("annotations")
    ]
    return {
        # create the format that the model expects
        "image": image,
        "instances": utils.annotations_to_instances(annos, image.shape[1:])
    }

And then, follwing this i overwrited the build_{train,test}_loader method

class MyTrainer(DefaultTrainer):
    @classmethod
    def build_train_loader(cls, cfg):
        train_dataloader =  build_detection_train_loader(cfg, mapper=custom_mapper)
        return train_dataloader

    @classmethod
    def build_test_loader(cls, cfg):
        test_dataloader = build_detection_train_loader(cfg, mapper=custom_mapper)
        return test_dataloader

I can see information of my dataset whether i print the {train,test}_dataloader variable. So all seems right.
Then i get this error when start to train

w, h = d["width"], d["height"]
KeyError: 'width'

Of course this does not work because instance.py creates a Instance as:

def __str__(self) -> str:
        s = self.__class__.__name__ + "("
        s += "num_instances={}, ".format(len(self))
        s += "image_height={}, ".format(self._image_size[0])
        s += "image_width={}, ".format(self._image_size[1])
        s += "fields=[{}])".format(", ".join((f"{k}: {v}" for k, v in self._fields.items())))
        return s

So, need also create a custom Instance where image_width == width and image_height == height ?