Small API cleanup to Cityscapes (#725)

Author: TheCodez

torchvision/datasets/cityscapes.py

@@ -26,49 +26,49 @@ class Cityscapes(data.Dataset):
         Get semantic segmentation target
 
         .. code-block:: python
-            dataset = Cityscapes('./data/cityscapes', split='train', mode='gtFine',
+            dataset = Cityscapes('./data/cityscapes', split='train', mode='fine',
                                  target_type='semantic')
 
             img, smnt = dataset[0]
 
         Get multiple targets
 
         .. code-block:: python
-            dataset = Cityscapes('./data/cityscapes', split='train', mode='gtFine',
+            dataset = Cityscapes('./data/cityscapes', split='train', mode='fine',
                                  target_type=['instance', 'color', 'polygon'])
 
             img, (inst, col, poly) = dataset[0]
 
-        Validate on the "gtCoarse" set
+        Validate on the "coarse" set
 
         .. code-block:: python
-            dataset = Cityscapes('./data/cityscapes', split='val', mode='gtCoarse',
+            dataset = Cityscapes('./data/cityscapes', split='val', mode='coarse',
                                  target_type='semantic')
 
             img, smnt = dataset[0]
     """
 
-    def __init__(self, root, split='train', mode='gtFine', target_type='instance',
+    def __init__(self, root, split='train', mode='fine', target_type='instance',
                  transform=None, target_transform=None):
         self.root = os.path.expanduser(root)
+        self.mode = 'gtFine' if mode == 'fine' else 'gtCoarse'
         self.images_dir = os.path.join(self.root, 'leftImg8bit', split)
-        self.targets_dir = os.path.join(self.root, mode, split)
+        self.targets_dir = os.path.join(self.root, self.mode, split)
         self.transform = transform
         self.target_transform = target_transform
         self.target_type = target_type
         self.split = split
-        self.mode = mode
         self.images = []
         self.targets = []
 
-        if mode not in ['gtFine', 'gtCoarse']:
-            raise ValueError('Invalid mode! Please use mode="gtFine" or mode="gtCoarse"')
+        if mode not in ['fine', 'coarse']:
+            raise ValueError('Invalid mode! Please use mode="fine" or mode="coarse"')
 
-        if mode == 'gtFine' and split not in ['train', 'test', 'val']:
-            raise ValueError('Invalid split for mode "gtFine"! Please use split="train", split="test"'
+        if mode == 'fine' and split not in ['train', 'test', 'val']:
+            raise ValueError('Invalid split for mode "fine"! Please use split="train", split="test"'
                              ' or split="val"')
-        elif mode == 'gtCoarse' and split not in ['train', 'train_extra', 'val']:
-            raise ValueError('Invalid split for mode "gtCoarse"! Please use split="train", split="train_extra"'
+        elif mode == 'coarse' and split not in ['train', 'train_extra', 'val']:
+            raise ValueError('Invalid split for mode "coarse"! Please use split="train", split="train_extra"'
                              ' or split="val"')
 
         if not isinstance(target_type, list):