TensorFlow XLM doesn't accept NumPy arrays for the attention mask
Convert NumPy attention mask to a TensorFlow tensor so that the mask creation doesn't crash

closes #2729 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=h1) Report
> Merging [#2736](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9c67196b83a824df577742d32d38e9121d8a9285?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2736/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2736      +/-   ##
==========================================
+ Coverage   74.09%   74.09%   +<.01%     
==========================================
  Files          93       93              
  Lines       15249    15251       +2     
==========================================
+ Hits        11298    11300       +2     
  Misses       3951     3951
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2736/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `90.47% <100%> (+0.06%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=footer). Last update [9c67196...3c9a47e](https://codecov.io/gh/huggingface/transformers/pull/2736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi, any update on this PR?<|||||>After discussing it with @thomwolf, it seems I was mistaken when believing that our TensorFlow models should accept numpy inputs. They should be converted to TensorFlow inputs. We should update the documentation to reflect this. Closing this PR as unrelated to the doc changes.