pass correct head mask to cross-attention layer
# What does this PR do?


 MBart, Blender, and Pegaus models decoder layers pass `layer_head_mask` head mask to cross attention layer, which is incorrect. This PR passes the correct `encoder_layer_head_mask` to cross-attention layer.
I see, then this looks a bit confusing to me, other seq2seq models (BART, Marian, BlendSmall) pass `encoder_layer_head_mask` to cross attention

https://github.com/huggingface/transformers/blob/9248e27037ce7f7c9359802e6fdf819a1e227a18/src/transformers/models/bart/modeling_bart.py#L419

https://github.com/huggingface/transformers/blob/9248e27037ce7f7c9359802e6fdf819a1e227a18/src/transformers/models/marian/modeling_marian.py#L438 

https://github.com/huggingface/transformers/blob/9248e27037ce7f7c9359802e6fdf819a1e227a18/src/transformers/models/blenderbot_small/modeling_blenderbot_small.py#L421<|||||>> s looks a bit confusing to me, other seq2seq models (BART, Marian, BlendSmall) pass `encoder_layer_head_mask` to cross attention

I would argue that they are wrong then :D (cc @stancld - maybe do you have more insight here?)<|||||>@patil-suraj @patrickvonplaten - What a coincidence... I was discussing this topic with my PhD supervisor this morning and I think the most proper way how to handle head masking for cross attention is to introduce a separate cross-attention head mask tensor to disentangle the cross-attention effect from the self-attention one?<|||||>I guess this would actually be the cleanest option! At the moment the cross-attention layer the exact same shape as the decoder self-attention layer -> so I think we can use the same mask for now and maybe improve it later with a `cross-attention head mask`. Using the `encoder_layer_head_mask` however could lead to errors IMO - so this option is just wrong to me<|||||>I can create a new issue and can have a look at this cross-attention `head_mask` at the weekend :)<|||||>That would be great @stancld! I will close this PR.