OpenAI GPT NoPaddingTokenFastTokenizerMatchingTest test fails
# 🐛 Bug

## Information

`tests/test_tokenization_fast.py::NoPaddingTokenFastTokenizerMatchingTest::test_all_tokenizers`
fails

## To reproduce

```
pytest   -n 1 tests/test_tokenization_fast.py::NoPaddingTokenFastTokenizerMatchingTest::test_all_tokenizers
================================================================ test session starts =================================================================
platform linux -- Python 3.7.5, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /mnt/nvme1/code/huggingface/transformers-master
plugins: hypothesis-5.5.4, filter-subpackage-0.1.1, arraydiff-0.3, flaky-3.6.1, ipynb-1.1.1.dev0, cov-2.10.0, astropy-header-0.1.2, forked-1.2.0, doctestplus-0.5.0, openfiles-0.4.0, remotedata-0.3.2, xdist-1.32.0
gw0 [1]
F                                                                                                                                              [100%]
====================================================================== FAILURES ======================================================================
____________________________________________ NoPaddingTokenFastTokenizerMatchingTest.test_all_tokenizers _____________________________________________
[gw0] linux -- Python 3.7.5 /home/stas/anaconda3/envs/main/bin/python

self = <tests.test_tokenization_fast.NoPaddingTokenFastTokenizerMatchingTest testMethod=test_all_tokenizers>

    def test_all_tokenizers(self):
        for tok_case in self.TOKENIZERS_CLASSES:
            for pretrained_name in tok_case.python_cls.pretrained_vocab_files_map[tok_case.vocab_key].keys():
    
                # Tokenizer.filter makes it possible to filter which Tokenizer to case based on all the
                # information available in Tokenizer (name, rust class, python class, vocab key name)
                if tok_case.filter is None or (
                    tok_case.filter is not None and tok_case.filter(tok_case, pretrained_name)
                ):
                    kwargs = dict(t for t in tok_case.kwargs) if tok_case.kwargs else {}
                    with self.subTest("{} ({})".format(tok_case.name, pretrained_name)):
                        tokenizer_r = tok_case.rust_cls.from_pretrained(pretrained_name, **kwargs)
                        tokenizer_p = tok_case.python_cls.from_pretrained(pretrained_name, **kwargs)
    
>                       self.fast_align_python(tokenizer_r, tokenizer_p, tok_case, pretrained_name)

tests/test_tokenization_fast.py:62: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_tokenization_fast.py:836: in fast_align_python
    self.assert_tokenization_python_rust_equals(tokenizer_r, tokenizer_p)
tests/test_tokenization_fast.py:192: in assert_tokenization_python_rust_equals
    self.assertSequenceEqual(input_p[key], input_r[key])
E   AssertionError: Sequences differ: [616,[111 chars] 0, 40477, 4830, 994, 580, 566, 260, 5958, 260[5290 chars] 239] != [616,[111 chars] 0, 4830, 994, 580, 566, 260, 5958, 260, 1490,[5160 chars] 239]
E
E   First differing element 29:
E   40477
E   4830
E
E   First sequence contains 12 additional elements.
E   First extra element 973:
E   1832
E
E   Diff is 8701 characters long. Set self.maxDiff to None to see it.
----------------------------------------------------------------- Captured log call ------------------------------------------------------------------
WARNING  transformers.tokenization_utils_base:tokenization_utils_base.py:2086 Token indices sequence length is longer than the specified maximum sequence length for this model (985 > 512). Running this sequence through the model will result in indexing errors
================================================================== warnings summary ==================================================================
/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/tensorflow_core/python/pywrap_tensorflow_internal.py:15
  /home/stas/anaconda3/envs/main/lib/python3.7/site-packages/tensorflow_core/python/pywrap_tensorflow_internal.py:15: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/directives.py:55
  /home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/directives.py:55: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    assert isinstance(locations, collections.Iterable), 'Must provide locations for directive.'

/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/directives.py:62
  /home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/directives.py:62: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    assert isinstance(args, collections.Mapping), '{} args must be a dict with argument names as keys.'.format(name)

/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/typemap.py:1
  /home/stas/anaconda3/envs/main/lib/python3.7/site-packages/graphql/type/typemap.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    from collections import OrderedDict, Sequence, defaultdict

-- Docs: https://docs.pytest.org/en/latest/warnings.html
============================================================== short test summary info ===============================================================
FAILED tests/test_tokenization_fast.py::NoPaddingTokenFastTokenizerMatchingTest::test_all_tokenizers - AssertionError: Sequences differ: [616,[111 ...
=========================================================== 1 failed, 4 warnings in 5.44s ============================================================
```

## Expected behavior

not fail ;)

I started digging and it fails only for `OpenAI GPT` tokenizer and passes `GPT2` tests.

If someone could give me a pointer, I'd be happy to investigate more. 

I'm not sure whether it's the python or rust implementation that is at fault.

I tested that if I force max_length=512, to remove the warning, the test still fails in the same way.

## Environment info

```
- `transformers` version: 3.0.2
- tokenizers: 0.8.1rc1
- Platform: Linux-4.15.0-109-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.5
- PyTorch version (GPU?): 1.5.1 (True)
- Tensorflow version (GPU?): 2.0.1 (False)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>
```


This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>looks like it has been fixed.