Nicer error message when Bert's input is missing batch size
Currently it fails in the computation of the attention_mask.

Let's fail with a shape error message instead.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2068?src=pr&el=h1) Report
> Merging [#2068](https://codecov.io/gh/huggingface/transformers/pull/2068?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2d5d86e03779b4b316698438caff0f675ee54abd?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2068/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2068?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2068      +/-   ##
==========================================
+ Coverage   83.15%   83.17%   +0.01%     
==========================================
  Files         109      109              
  Lines       15869    15869              
==========================================
+ Hits        13196    13199       +3     
+ Misses       2673     2670       -3
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2068?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2068/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `87.68% <100%> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2068/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2NvbW1vbl90ZXN0LnB5) | `74.23% <0%> (+0.57%)` | :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/2068?src=pr&el=footer). Last update [2d5d86e...18fb935](https://codecov.io/gh/huggingface/transformers/pull/2068?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>lgtm<|||||>should this fix be added here as well?  @thomwolf https://github.com/huggingface/transformers/blob/6cc06d17394f5715cdf2d13a1ef7680bedaee9e2/src/transformers/modeling_utils.py#L700