Bump xformers for compatibility with torch (#7022)

## Summary

#6890 bumped torch, which caused an incompatibility with xformers when
installing with `pip install ".[xformers]"`. This PR bumps xformers.

## QA Instructions

I ran some smoke tests to confirm that generating with xformers still
works.

In my tests on an A100, there is a performance regression after bumping
xformers (2.7 it/s vs 3.2 it/s). I think it is ok to ignore this for
A100s, since users should be using torch-sdp, which is much faster (4.3
it/s). But, we should test for regression on older cards where xformers
is still recommended.

## Checklist

- [x] _The PR has a short but descriptive title, suitable for a
changelog_
- [x] _Tests added / updated (if applicable)_
- [x] _Documentation added / updated (if applicable)_

pyproject.toml

@@ -101,7 +101,7 @@ dependencies = [
 [project.optional-dependencies]
 "xformers" = [
   # Core generation dependencies, pinned for reproducible builds.
-  "xformers==0.0.25post1; sys_platform!='darwin'",
+  "xformers==0.0.28.post1; sys_platform!='darwin'",
   # Auxiliary dependencies, pinned only if necessary.
   "triton; sys_platform=='linux'",
 ]