Fixes EMNIST classes attribute is wrong #1716 (#1736)

* Fixes #1716

Fixes EMNIST classes attribute is wrong #1716

* Fixed the Classes for Letters Split

* Update mnist.py

* Move classes attribute inside init definition

* Fix Linting errors

torchvision/datasets/mnist.py

@@ -7,6 +7,7 @@ import os.path
 import numpy as np
 import torch
 import codecs
+import string
 from .utils import download_url, download_and_extract_archive, extract_archive, \
     makedir_exist_ok, verify_str_arg
 
@@ -239,12 +240,24 @@ class EMNIST(MNIST):
     url = 'http://www.itl.nist.gov/iaui/vip/cs_links/EMNIST/gzip.zip'
     md5 = "58c8d27c78d21e728a6bc7b3cc06412e"
     splits = ('byclass', 'bymerge', 'balanced', 'letters', 'digits', 'mnist')
+    # Merged Classes assumes Same structure for both uppercase and lowercase version
+    _merged_classes = set(['C', 'I', 'J', 'K', 'L', 'M', 'O', 'P', 'S', 'U', 'V', 'W', 'X', 'Y', 'Z'])
+    _all_classes = set(list(string.digits + string.ascii_letters))
+    classes_split_dict = {
+        'byclass': list(_all_classes),
+        'bymerge': sorted(list(_all_classes - _merged_classes)),
+        'balanced': sorted(list(_all_classes - _merged_classes)),
+        'letters': list(string.ascii_lowercase),
+        'digits': list(string.digits),
+        'mnist': list(string.digits),
+    }
 
     def __init__(self, root, split, **kwargs):
         self.split = verify_str_arg(split, "split", self.splits)
         self.training_file = self._training_file(split)
         self.test_file = self._test_file(split)
         super(EMNIST, self).__init__(root, **kwargs)
+        self.classes = self.classes_split_dict[self.split]
 
     @staticmethod
     def _training_file(split):