BERT decoder: Fix failure with the default attention mask.
PyTorch < 1.3 requires multiplication operands to be of the same type. This was violated when using default attention mask (i.e.., `attention_mask=None` in arguments) given BERT in the decoder mode.

In particular, this was breaking `Model2Mode`l and made a tutorial from quickstart.md failing.

A test is included, but here is a minimal snippet to reproduce:

```python
import torch
from transformers import BertModel
model = BertModel.from_pretrained("bert-base-uncased", is_decoder=True)
inputs = torch.LongTensor([[1, 2, 3]])
model(inputs)    # no `attention_mask` provided
```

On PyTorch 1.2 or older this was failing with 
```
Traceback (most recent call last):
...
  File "/home/oleksiy.syvokon/transformers/src/transformers/modeling_bert.py", line 735, in forward
    extended_attention_mask = causal_mask[:, None, :, :] * attention_mask[:, None, None, :]
RuntimeError: expected device cpu and dtype Float but got device cpu and dtype Long
```
Thanks for the feedback! That's a valid concern. I made config handling consistent, at least for the BERT tests. But if you decide that it's too much change for such a trivial fix, I can revert the changes in tests.<|||||>(Seems like CircleCI tests failure is transient and unrelated to the PR)<|||||>That's one way of solving the issue, but now it makes the BERT tests incoherent with the rest of the tests, which all use tuples instead of dictionaries. For this PR, I believe the most simple would be to revert to using tuples and use this tuple in `test_bert_model_as_decoder_with_default_input_mask`. What do you think?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2756?src=pr&el=h1) Report
> Merging [#2756](https://codecov.io/gh/huggingface/transformers/pull/2756?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1f5db9a13c8932e02e6e7d599a16dc262b1570bf?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/2756/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2756?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2756   +/-   ##
=======================================
  Coverage   75.02%   75.02%           
=======================================
  Files          93       93           
  Lines       15275    15275           
=======================================
  Hits        11460    11460           
  Misses       3815     3815
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2756?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.9% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9jYW1lbWJlcnQucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG1fcm9iZXJ0YS5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9jdHJsLnB5) | `97.82% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `96.54% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `96.05% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.84% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `95.11% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `94.66% <0%> (ø)` | :arrow_up: |
| ... and [19 more](https://codecov.io/gh/huggingface/transformers/pull/2756/diff?src=pr&el=tree-more) | |

------

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<|||||>@LysandreJik, I agree. Reverted unnecessary changes in tests.<|||||>Great, thanks @asivokon !!