[fix] Add back BlockDiagonalMask import

[tanvitiwari-meta]

It looks like BlockDiagonalMask import was accidentally removed in commit 166fd2e. This commit adds it back.

What does this PR do?

Fixes # (issue).

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  • I work at Meta; I discussed this change with xformer maintainer over chat.
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PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

[bottler]

I think the change may have deliberate. It can be accessed as fmha.attn_bias.BlockDiagonalMask just like other attn_bias objects.

[danthe3rd]

Apparently this is breaking some projects. So I'm fine with putting it back there.
However we need to fix the linter - let's just add it to the __all__ enumeration :)

[danthe3rd]

Thanks!