Remove dependency on pytest for running tests (#2055)

* Switch to plain unittest for skipping slow tests.

Add a RUN_SLOW environment variable for running them.

* Switch to plain unittest for PyTorch dependency.

* Switch to plain unittest for TensorFlow dependency.

* Avoid leaking open files in the test suite.

This prevents spurious warnings when running tests.

* Fix unicode warning on Python 2 when running tests.

The warning was:

    UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal

* Support running PyTorch tests on a GPU.

Reverts 27e015bd.

* Tests no longer require pytest.

* Make tests pass on cuda

transformers/tokenization_gpt2.py

@@ -72,7 +72,7 @@ def bytes_to_unicode():
     """
     Returns list of utf-8 byte and a mapping to unicode strings.
     We specifically avoids mapping to whitespace/control characters the bpe code barfs on.
-    
+
     The reversible bpe codes work on unicode strings.
     This means you need a large # of unicode characters in your vocab if you want to avoid UNKs.
     When you're at something like a 10B token dataset you end up needing around 5K for decent coverage.
@@ -122,13 +122,15 @@ class GPT2Tokenizer(PreTrainedTokenizer):
         self.max_len_single_sentence = self.max_len # no default special tokens - you can update this value if you add special tokens
         self.max_len_sentences_pair = self.max_len # no default special tokens - you can update this value if you add special tokens
 
-        self.encoder = json.load(open(vocab_file, encoding="utf-8"))
+        with open(vocab_file, encoding="utf-8") as vocab_handle:
+            self.encoder = json.load(vocab_handle)
         self.decoder = {v: k for k, v in self.encoder.items()}
         self.errors = errors  # how to handle errors in decoding
         self.byte_encoder = bytes_to_unicode()
         self.byte_decoder = {v: k for k, v in self.byte_encoder.items()}
-        bpe_data = open(merges_file, encoding='utf-8').read().split('\n')[1:-1]
-        bpe_merges = [tuple(merge.split()) for merge in bpe_data]
+        with open(merges_file, encoding='utf-8') as merges_handle:
+            bpe_merges = merges_handle.read().split('\n')[1:-1]
+        bpe_merges = [tuple(merge.split()) for merge in bpe_merges]
         self.bpe_ranks = dict(zip(bpe_merges, range(len(bpe_merges))))
         self.cache = {}
 
@@ -234,4 +236,4 @@ class GPT2Tokenizer(PreTrainedTokenizer):
                 writer.write(' '.join(bpe_tokens) + u'\n')
                 index += 1
 
-        return vocab_file, merge_file
+        return vocab_file, merge_file