Flexible and easy to use HSDP setting

[Liyang90]

What does this PR do?

Fixes #19502

It allows users to provide device_mesh as a tuple to the FSDPStrategy.

The test case for hybrid fsdp strategy would be broken, because _init_sharding_strategy is moved out of the __init__ function.

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📚 Documentation preview 📚: https://pytorch-lightning--19504.org.readthedocs.build/en/19504/

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 58%. Comparing base (76b691d) to head (9d7dfbe).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #19504     +/-   ##
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- Coverage      82%      58%    -24%     
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  Files         424      419      -5     
  Lines       34919    34852     -67     
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- Hits        28628    20127   -8501     
- Misses       6291    14725   +8434     

[awaelchli]

Great job. Looking forward to this feature.

[awaelchli]

Since the tuple specification is a feature of Lightning, we should list the device_mesh parameter explicitly in the init args (see the docstring I added). The kwargs are for things that we pass down to FSDP directly.

So I suggest to set self.device_mesh and update this attribute here to the actual DeviceMesh 😃

[Liyang90]

If device_mesh is separated from kwargs, we will fail the check in _init_sharding_strategy in the self.__init__.

[Liyang90]

Changed now. @awaelchli could you take a look again?

[npuichigo]

Any updates on this?