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/