Document the dependcy on datasets
Author: sgugger
Pull request: yes

We need the latest version of datasets to run the tests (since Trainer uses it). When datasets is more settled and we don't necessarily need to quickly update it, we can change this to a regular dep in "dev" with a minimal version pinned.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7058?src=pr&el=h1) Report
> Merging [#7058](https://codecov.io/gh/huggingface/transformers/pull/7058?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/514486739cc732ad05549d81bd48c0aa9e03a0f3?el=desc) will **increase** coverage by `0.14%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7058/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7058?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7058      +/-   ##
==========================================
+ Coverage   79.32%   79.47%   +0.14%     
==========================================
  Files         168      168              
  Lines       32172    32172              
==========================================
+ Hits        25522    25568      +46     
+ Misses       6650     6604      -46     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7058?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.91% <0.00%> (-0.14%)` | :arrow_down: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `94.44% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `94.00% <0.00%> (+4.00%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `93.54% <0.00%> (+5.37%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7058/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+7.51%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7058?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/7058?src=pr&el=footer). Last update [5144867...d360cf8](https://codecov.io/gh/huggingface/transformers/pull/7058?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).