Fabric: Enable "auto" for devices and accelerator as cli arguments

[fnhirwa]

What does this PR do?

Fixes #20451

This PR adds support for the auto variable for accelerator and devices in the Fabric CLI.

• When auto is passed to --accelerator we now detect whether the machine has either mps, cuda, or cpu and dynamically runs on the available device, this is the same case when no accelerator is specified.

• For auto in the --devices argument the devices default to 1 as specified in the docstring of the argument here

  pytorch-lightning/src/lightning/fabric/cli.py

  Line 80 in c03660a

  default="1",

No breaking changes were introduced.

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---

📚 Documentation preview 📚: https://pytorch-lightning--20913.org.readthedocs.build/en/20913/

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87%. Comparing base (3138305) to head (4f88200).
⚠️ Report is 194 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20913   +/-   ##
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- Coverage      87%      87%   -0%     
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  Files         268      268           
  Lines       23442    23453   +11     
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+ Hits        20394    20401    +7     
- Misses       3048     3052    +4