simplify read_image_file and read_label_file functions (#2476)

Summary:
Pull Request resolved: #2476

I simplified the read_image_file and read_label_file functions to make it easier to adapt this code to use it with other file systems that aren't compatible with the simple open command.

Reviewed By: fmassa

Differential Revision: D22530585

fbshipit-source-id: 0dd20f917c947d1ec1eae23474c251ceea85f427

Author: ajaech

torchvision/datasets/mnist.py

@@ -470,16 +470,14 @@ def read_sn3_pascalvincent_tensor(path, strict=True):
 
 
 def read_label_file(path):
-    with open(path, 'rb') as f:
-        x = read_sn3_pascalvincent_tensor(f, strict=False)
+    x = read_sn3_pascalvincent_tensor(path, strict=False)
     assert(x.dtype == torch.uint8)
     assert(x.ndimension() == 1)
     return x.long()
 
 
 def read_image_file(path):
-    with open(path, 'rb') as f:
-        x = read_sn3_pascalvincent_tensor(f, strict=False)
+    x = read_sn3_pascalvincent_tensor(path, strict=False)
     assert(x.dtype == torch.uint8)
     assert(x.ndimension() == 3)
     return x