create encoder attention mask from shape of hidden states
As noted by @efeiefei (#1770) we currently create masks on the encoder hidden states (when they're not provided) based on the shape of the inputs to the decoder. This is obviously wrong; sequences can be of different lengths. We now create the encoder attention mask based on the `batch_size` and `sequence_length` of the encoder hidden states.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2107?src=pr&el=h1) Report
> Merging [#2107](https://codecov.io/gh/huggingface/transformers/pull/2107?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0cb163865a4c761c226b151283309eedb2b1ca4d?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/2107/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2107?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2107      +/-   ##
==========================================
+ Coverage   82.67%   82.67%   +<.01%     
==========================================
  Files         111      111              
  Lines       16162    16164       +2     
==========================================
+ Hits        13362    13364       +2     
  Misses       2800     2800
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2107?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2107/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `87.72% <100%> (+0.04%)` | :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/2107?src=pr&el=footer). Last update [0cb1638...3520be7](https://codecov.io/gh/huggingface/transformers/pull/2107?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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