Make create_extended_attention_mask_for_decoder static method
Author: pbelevich
Pull request: yes

# What does this PR do?'

`create_extended_attention_mask_for_decoder` doesn't access `self` and can be `@staticmethod`. This resolves some issues with fx tracing for pytorch pipeline parallelism project. 

cc @michaelbenayoun

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

(just a comment)

Would it be possible to provide the code sample for the issue that occurs without this PR, or a link to the issue page?

Looks good to me once all the tests pass.
Pinging @sgugger for review!

> Would it be possible to provide the code sample for the issue that occurs without this PR, or a link to the issue page?

The project will be released and the repo will be opened soon

> Looks good to me once all the tests pass.

@michaelbenayoun @sgugger all tests passed

Thanks again for your contribution!