Warning on eval module

[SkafteNicki]

What does this PR do?

Fixes #20986
Fixes #20216
Fixes #20573
Fixes #20177

Adds a simple warning if any module when fit starts are in eval mode. There are cases where you intentionally want modules in eval mode even during fit, however most users want things to be in train mode. This warning should hopefully help users identify this can be a potential problem.

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📚 Documentation preview 📚: https://pytorch-lightning--21146.org.readthedocs.build/en/21146/