Catch issue with partial mpi4py installation.

[roccomoretti]

What does this PR do?

Depending on precise install conditions, mpi4py may be installed without MPI being present, which can cause an ImportError. ("cannot import name 'MPI' from 'mpi4py'").

Catch that case in the detect() function of the MPI plugin, and treat it as if mpi4py is not installed.

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📚 Documentation preview 📚: https://pytorch-lightning--21353.org.readthedocs.build/en/21353/

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87%. Comparing base (c76cec6) to head (c432e9f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21353   +/-   ##
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- Coverage      87%      87%   -0%     
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  Files         269      269           
  Lines       23796    23799    +3     
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+ Hits        20617    20618    +1     
- Misses       3179     3181    +2     

[justusschock]

@roccomoretti , thanks for this PR. Could you please add a section to the Changelog?

[roccomoretti]

@justusschock Where is the Changelog? There doesn't seem to be a changelog.md or the like in the root directory, nor is there any link or mention of the changelog in the contributor guidelines.

[justusschock]

@roccomoretti apologies for being less clear. I'll also update the contributor guidelines. For fabric (where you updated things) it's here: https://github.com/Lightning-AI/pytorch-lightning/blob/master/src/lightning/fabric/CHANGELOG.md

[deependujha]

Hi @roccomoretti, I believe it makes sense to add a brief comment on why this try-except block is required here, for future reference.