Reduce number of times optimizers are instantiated with FSDP

[ananthsub]

What does this PR do?

Follow up to comment here: #11952 (comment)

Currently we are calling LightningModule.configure_optimizers 3 times in the FSDP strategy 😞
We only need to do this one, after the model is wrapped with FSDP inside of model_sharded_context

Does your PR introduce any breaking changes? If yes, please list them.

No

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