Check for DTensor due to FSDP2 in OptimizerMonitor

[aadyotb]

What does this PR do?

This PR fixes a bug with the OptimizerMonitor being used in conjunction with FSDP2. Specifically, FSDP2 causes some of the optimizer metrics to be DTensor objects, which have issues being all-reduced. This PR converts the DTensors to local tensors before doing any further operations.

What issue(s) does this change relate to?

Fixes #3920

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