Only self-attention returned when output_attentions = True for BERT
# 🐛 Bug
```
if self.output_attentions:
                all_attentions = all_attentions + (layer_outputs[1],)
```
should be
```
if self.output_attentions:
                all_attentions = all_attentions + (layer_outputs[1:],)
```
## Information
Line 412 [here](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_bert.py
)

I assume that there is no reason to exclude cross-attention from all_attentions. 

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