[EncoderDecoder] Add Cross Attention for GPT2
This PR implements **Bert2GPT2** by adding cross-attention layers to GPT2. 

Note that currently it is not possible to speed up decoder generation with the encoder-decoder framework (by using GPT2's past tensors) since it has to be implemented for all models that are compatible with the encoder/decoder framework (Bert, Roberta) before it can be used within the framework.

All GPT2 `RUN_SLOW` tests are verified to pass.

**Future PRs TODO**:
- [ ] Verify that Bert2GPT2 works by training on CNN Daily Mail summarization
- [ ] Add smart caching to Bert and add it to the encoder-decoder framework
- [ ] Update encoder-decoder docs
- [ ] Add a notebook explaining how to use encoder-decoder models.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=h1) Report
> Merging [#6415](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bc820476a5c72060f810f825298befd5ec85da4d&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `96.61%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6415/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6415      +/-   ##
==========================================
- Coverage   79.98%   79.98%   -0.01%     
==========================================
  Files         153      153              
  Lines       28005    28039      +34     
==========================================
+ Hits        22401    22427      +26     
- Misses       5604     5612       +8     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `91.66% <87.50%> (+0.64%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.68% <97.87%> (+0.71%)` | :arrow_up: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `96.94% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.47% <0.00%> (-32.95%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+7.51%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6415/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `87.73% <0.00%> (+63.19%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=footer). Last update [bc82047...56094e2](https://codecov.io/gh/huggingface/transformers/pull/6415?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
