bug: fsdp cannot load optimizor state using dcp

[flymin]

What does this PR do?

Fixes # (3896) partially. Fix bug when using dcp, the optimizor state cannot be loaded. Tested with fsdp2.

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• Did you write any new necessary tests?

Who can review?

@SunMarc

[SunMarc]

Thanks ! Can you add a test to check that it works ?

[flymin]

I am not familiar with how testing system works. Could anyone else offer some help? The logic should be straightforward. Before this PR, nothing would be loaded from the optimizer checkpoint.

[SunMarc]

It's fine, we will rework on the tests at some point ! Code looks good