FSDP (native) support for LightningLite

[awaelchli]

What does this PR do?

Adds support for strategy="fsdp" in LightningLite.

Fixes #14898

TODOs:

• no sync after Efficient gradient accumulation in LightningLite #14966 (see also this and this)

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @carmocca @justusschock @awaelchli

[tchaton]

LGTM !