head mask issue  transformers==3.5.1
## Environment info
     
- `transformers` version: 3.5.1
- Platform: windows & linux
- Python version: python 3.7
- PyTorch version (GPU?): 1.7.0  
- Tensorflow version (GPU?): 2.3.1
- Using GPU in script?: Yes  both CPU and GPU
- Using distributed or parallel set-up in script?: No

### Who can help
albert, bert, GPT2, XLM: @LysandreJik 

## Information
Hi 
I am using  tiny albert Chinese as an encoder. and I've also tried to use albert transformer in my code.
Thing is I have to change  a little bit source code to avoid some head mask issues.
watch transformers/modeling_albert.py 
line 387:  layer_output = albert_layer(hidden_states, attention_mask, **head_mask[layer_index]**, output_attentions)
however a few lines above, the defualt head_mask is None. So **TypeError: 'NoneType' object is not subscriptable** would be raised

It's not a deep bug and could be easily avoided if making a torch.ones  head_mask. Just want to bring it up so it might probably help the others who encounter the same problem.  

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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