Adding gradient checkpointing to GPT2
This PR adds gradient checkpointing capabilities to GPT-2, imitating the Longformer and Bert checkpointing code. It also disables `find_unused_parameters` in Trainer if the model is using gradient checkpointing, as per #4659 they are incompatible.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7446?src=pr&el=h1) Report
> Merging [#7446](https://codecov.io/gh/huggingface/transformers/pull/7446?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7dfdf793bb5e3a865f33ed597b10fc4526364af9?el=desc) will **decrease** coverage by `1.92%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7446/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7446?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7446      +/-   ##
==========================================
- Coverage   80.98%   79.06%   -1.93%     
==========================================
  Files         181      181              
  Lines       35750    35757       +7     
==========================================
- Hits        28953    28271     -682     
- Misses       6797     7486     +689     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7446?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `55.70% <ø> (ø)` | |
| [src/transformers/configuration\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2dwdDIucHk=) | `97.36% <100.00%> (+0.07%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `87.03% <100.00%> (+0.20%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `22.14% <0.00%> (-72.41%)` | :arrow_down: |
| [src/transformers/modeling\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yYWcucHk=) | `25.39% <0.00%> (-51.59%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `65.26% <0.00%> (-33.64%)` | :arrow_down: |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `60.00% <0.00%> (-30.00%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `74.14% <0.00%> (-18.70%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `79.16% <0.00%> (-4.17%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `90.12% <0.00%> (-3.79%)` | :arrow_down: |
| ... and [8 more](https://codecov.io/gh/huggingface/transformers/pull/7446/diff?src=pr&el=tree-more) | |

------

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<|||||>The slow tests are passing - I've also added a test for generation with checkpointing, although of course to be sure, one should also check the contents of the backwards pass.