include tf gpt2 tests for attn mask and past variable
Test TF GPT2 for correct behavior regarding the past and attn mask variable. Translated code from torch to TF 2.0. 


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=h1) Report
> Merging [#3122](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/34de670dbe70a9ead31d0692ad9dc726d3ea4edb?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3122/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #3122      +/-   ##
=========================================
- Coverage   77.84%   77.8%   -0.05%     
=========================================
  Files          98      98              
  Lines       16422   16422              
=========================================
- Hits        12784   12777       -7     
- Misses       3638    3645       +7
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3122/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.87% <0%> (-0.86%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3122/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.45% <0%> (-0.16%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=footer). Last update [34de670...1305f35](https://codecov.io/gh/huggingface/transformers/pull/3122?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
