[WIP] Language modeling example for TF Trainer
To support language modeling for TF Trainer.


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=h1) Report
> Merging [#6955](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c5d43a872f0e85ce069e921c5bda02374e5b9cbf?el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6955/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6955      +/-   ##
==========================================
- Coverage   80.02%   80.00%   -0.02%     
==========================================
  Files         161      161              
  Lines       30120    30120              
==========================================
- Hits        24104    24098       -6     
- Misses       6016     6022       +6     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6955/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.21% <0.00%> (-1.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6955/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6955/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.30% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=footer). Last update [c5d43a8...8e24159](https://codecov.io/gh/huggingface/transformers/pull/6955?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>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.
