Fix output name
Output variable name `all_hidden_states` found in README is inconsistent with documentation's `hidden_states`: https://huggingface.co/transformers/model_doc/bert.html#bertmodel
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2555?src=pr&el=h1) Report
> Merging [#2555](https://codecov.io/gh/huggingface/transformers/pull/2555?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6e2c28a14a3d171e8c4d3838429abb1d69456df5?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2555/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2555?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2555   +/-   ##
=======================================
  Coverage   74.66%   74.66%           
=======================================
  Files          87       87           
  Lines       14802    14802           
=======================================
  Hits        11052    11052           
  Misses       3750     3750
```



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