grammar corrections and train data update
- fixed grammar and spelling
- added an intro
- updated Training data references
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5448?src=pr&el=h1) Report
> Merging [#5448](https://codecov.io/gh/huggingface/transformers/pull/5448?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d16e36c7e525aab4c08a6e60a7478e209498dc14&el=desc) will **decrease** coverage by `0.86%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5448/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5448?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5448      +/-   ##
==========================================
- Coverage   77.82%   76.95%   -0.87%     
==========================================
  Files         141      141              
  Lines       24608    24608              
==========================================
- Hits        19150    18938     -212     
- Misses       5458     5670     +212     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5448?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.62% <0.00%> (-73.11%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.43% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.09% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `94.98% <0.00%> (+74.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5448?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/5448?src=pr&el=footer). Last update [d16e36c...20f340c](https://codecov.io/gh/huggingface/transformers/pull/5448?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
