Fix FSDP mixed precision semantics and add user warning

[deependujha]

What does this PR do?

Fixes #<issue_number>

Align FSDP "-mixed" precision modes with their "-true" counterparts, since parameters remain in full precision. Adds a warning for all reduced-precision modes indicating experimental status.

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📚 Documentation preview 📚: https://pytorch-lightning--21361.org.readthedocs.build/en/21361/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (8ac4843) to head (9323240).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21361   +/-   ##
=======================================
- Coverage      87%      87%   -0%     
=======================================
  Files         269      269           
  Lines       23799    23791    -8     
=======================================
- Hits        20618    20610    -8     
  Misses       3181     3181           

[deependujha]

doc failures are irrelevant and is due to

`sphinx.errors.SphinxWarning: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://docs.python.org/3/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 503 Server Error: Service Unavailable for url: https://docs.python.org/3/objects.inv

Warning, treated as error:
failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://docs.python.org/3/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 503 Server Error: Service Unavailable for url: https://docs.python.org/3/objects.inv
make: *** [Makefile:19: html] Error 2
`

seems something is down again.