Fix TensorFlow dataset generator
Author: jplu
Pull request: yes

Should fix #4856 

The method `glue_convert_examples_to_features` returns a badly formated TensorFlow dataset in case the used model doesn't use `token_type_ids` as feature such as DistilBert.

The fix is to detect when the feature `token_type_ids` should belong to the TensorFlow dataset or not. I'm not really happy of the fix, @julien-c and @LysandreJik do you have a better way to handle this?

Note: do not forget that the same fix should be applied for the other `xxx_examples_to_features` for other datasets processor in `src/data/processors`.

Comments:
Nice!! I didn't know that parameter ^^

Does it seems better now?

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4881?src=pr&el=h1) Report
> Merging [#4881](https://codecov.io/gh/huggingface/transformers/pull/4881?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/02e5f79662d72cccdca81a47e3001a5f6d36e5b1&el=desc) will **increase** coverage by `0.07%`.
> The diff coverage is `7.69%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4881/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4881?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4881      +/-   ##
==========================================
+ Coverage   76.46%   76.54%   +0.07%     
==========================================
  Files         128      128              
  Lines       21502    21511       +9     
==========================================
+ Hits        16442    16465      +23     
+ Misses       5060     5046      -14     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4881?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4881/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.21% <0.00%> (-0.45%)` | :arrow_down: |
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4881/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `49.09% <20.00%> (-0.36%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4881/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.27% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4881/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `94.73% <0.00%> (+10.04%)` | :arrow_up: |

------

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Ok, if this one is ok I'm gonna update the other methods the same way.

Should be ok now.

@julien-c @thomwolf @LysandreJik any issue to merge this?

@LysandreJik anything else to merge?