Add easy access to state_dict in Lite module wrapper

[awaelchli]

What does this PR do?

Fixes #14571

Breaking change

This is a breaking change for anyone who had to work-around the saving and loading with model.module, or mangling with the raw state-dict keys of model.state_dict().

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[Borda]

@awaelchli, mind checking/resolving the remaining conversations? 🦦