[fix] Torchscriptability FTW

[blefaudeux]

What does this PR do?

Fixes #246

Before submitting

• [🙃] Did you have fun?
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [o] Did you make sure to update the docs?
• [o] Did you write any new necessary tests?
  • there are tests existing for torchscript, but not on the whole model. Some parts will not work for sure (triton for instance).
• Did you update the changelog? (if needed)

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.

[blefaudeux]

cc @jramapuram

[blefaudeux]

follow the "if it looks like a duck and quack like a duck then it's a duck" python mantra. In any case @fmassa is rewriting this and it will disappear

[codecov-commenter]

Codecov Report

Merging #252 (490d4e7) into main (81eb650) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
- Coverage   92.21%   92.21%   -0.01%     
==========================================
  Files          60       60              
  Lines        3313     3312       -1     
==========================================
- Hits         3055     3054       -1     
  Misses        258      258              

Flag	Coverage Δ
Python	92.21% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
xformers/components/attention/attention_mask.py	98.48% <ø> (-0.03%)	⬇️

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