add gpt2-xl for tf
TF GPT2-XL is now added to AWS and can be loaded via:

```
from transformers import TFGPT2LMHeadModel
model = TFGPT2LMHeadModel.from_pretrained('gpt2-xl')
```

Thanks @bkkaggle for pointing this out! 
Good to merge for me. Tested whether model can generate text and everything seems fine.
@julien-c <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3268?src=pr&el=h1) Report
> Merging [#3268](https://codecov.io/gh/huggingface/transformers/pull/3268?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cc4c37952a961f2d13e83f3d5ba6dab811d0bbfd&el=desc) will **increase** coverage by `0.19%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3268/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3268?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3268      +/-   ##
==========================================
+ Coverage   77.82%   78.01%   +0.19%     
==========================================
  Files          98       98              
  Lines       16666    16666              
==========================================
+ Hits        12970    13002      +32     
+ Misses       3696     3664      -32     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3268?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3268/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `96.16% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3268/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.19% <0.00%> (+5.72%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3268?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/3268?src=pr&el=footer). Last update [cc4c379...501291a](https://codecov.io/gh/huggingface/transformers/pull/3268?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
