Make DataCollator a callable
Author: sgugger
Pull request: yes

As discussed with @julien-c, change the `DataCollator` to be a callable (and the trainer now can take any function as data collator).
I removed the abstract class and made a type alias to keep the type annotations instead.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=h1) Report
> Merging [#5015](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/66bcfbb130a3a60d873597fdca05a8d539ef3401&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `73.91%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5015/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5015      +/-   ##
==========================================
+ Coverage   77.20%   77.27%   +0.06%     
==========================================
  Files         128      128              
  Lines       21854    21845       -9     
==========================================
+ Hits        16873    16880       +7     
+ Misses       4981     4965      -16     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `89.65% <70.00%> (+0.42%)` | :arrow_up: |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <100.00%> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.43% <100.00%> (-0.05%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.26% <0.00%> (-0.16%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.61% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `78.65% <0.00%> (+2.29%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=footer). Last update [66bcfbb...88370ec](https://codecov.io/gh/huggingface/transformers/pull/5015?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


LGTM – we should however document this change in the next release notes, as it's breaking for end-users who would have implemented a custom data_collator