Add support for more dtypes in ModelSummary and warning on non-supported

[SkafteNicki]

What does this PR do?

Fixes #20598

Add support in ModelSummary for precision specifications like 32-true and not just the legacy options.
Additionally, add warning that the MB calculation will default to using 32bit when used with mixed precision types like 16-mixed which is not supported.

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📚 Documentation preview 📚: https://pytorch-lightning--21034.org.readthedocs.build/en/21034/