Provide access to unwrapped model in Lite

[awaelchli]

What does this PR do?

Fixes #12596

This PR enables attribute pass-through on the _LiteModule wrapper. When an attribute is not found on the wrapper, it tries to look it up on the original model that was passed into the setup method.

The _LiteModule wrapper now takes an optional argument original_module, which is the one passed in to the Lite.setup method. If we didn't do this, we wouldn't be able to access it, because the forward_module is potentially already wrapped by something like DistributedDataParallel.

Breaking change

This is a breaking change if you previously relied on LiteModule.state_dict() to save or load checkpoints. The old checkpoint won't load after this PR because of missing keys (_original_module, _forward_module).

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @tchaton @rohitgr7 @carmocca @justusschock @awaelchli

[akihironitta]

lgtm 🚀

[awaelchli]

Gatekeeper merge