Add head_mask and decoder_head_mask to PyTorch LED
This PR implements `head_mask` and `decoder_head_mask` for PyTorch LED (and Longformer as there's a copy dependency) and it is the follow-up to the open issue #9814. 

**Motivation:** This PR is a part of an endeavour to enable the usage of `head_mask` and `decoder_head_mask` for all encoder-decoder transformers following the recent work on BART-like models (#9569).

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Fixes: https://github.com/huggingface/transformers/issues/9814

Reviewers: @patrickvonplaten @LysandreJik @stas00 