adding positional embeds masking to TFRoBERTa
Author: thomwolf
Pull request: yes

Adding positional embeds masking to TFRoBERTa following its addition to the PT model in #1764 to fix PT <=> TF equivalence test

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=h1) Report
> Merging [#2254](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/73f6e9817c744caae0b73fa343ceaf95ba76f9f8?src=pr&el=desc) will **increase** coverage by `1.46%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2254/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2254      +/-   ##
==========================================
+ Coverage   77.28%   78.75%   +1.46%     
==========================================
  Files         133      131       -2     
  Lines       19872    19742     -130     
==========================================
+ Hits        15358    15547     +189     
+ Misses       4514     4195     -319
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3JvYmVydGEucHk=) | `90.9% <100%> (+0.47%)` | :arrow_up: |
| [transformers/modeling\_mmbt.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX21tYnQucHk=) | | |
| [transformers/configuration\_mmbt.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fbW1idC5weQ==) | | |
| [transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3BpcGVsaW5lcy5weQ==) | `67.94% <0%> (+0.58%)` | :arrow_up: |
| [transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3V0aWxzLnB5) | `64.29% <0%> (+0.71%)` | :arrow_up: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `81.51% <0%> (+1.32%)` | :arrow_up: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `96.47% <0%> (+2.2%)` | :arrow_up: |
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `73.72% <0%> (+2.29%)` | :arrow_up: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `64.42% <0%> (+10.09%)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `94.39% <0%> (+17.24%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/huggingface/transformers/pull/2254/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=footer). Last update [73f6e98...77676c2](https://codecov.io/gh/huggingface/transformers/pull/2254?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Yeepee!