Improve performance of available_accelerators by returning a set

[allrob23]

What does this PR do?

This PR updates available_accelerators to return a set instead of a list, improving performance for membership checks.

Using a set makes membership checks more efficient, reducing time complexity from O(n) to O(1) on average. This change enhances performance while maintaining functionality.

Fixes #20671

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

Reviewer checklist

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

---

📚 Documentation preview 📚: https://pytorch-lightning--20672.org.readthedocs.build/en/20672/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87%. Comparing base (df5dee6) to head (7666fd6).
Report is 130 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20672   +/-   ##
=======================================
- Coverage      87%      87%   -0%     
=======================================
  Files         268      268           
  Lines       23449    23449           
=======================================
- Hits        20501    20499    -2     
- Misses       2948     2950    +2