fix: add DefaultLoadPlanner to FSDP2 + SHARDED_STATE_DICT optimizer checkpoint loading

[iavinas]

Without the planner, dist_cp.load() cannot resolve nested optimizer state keys (e.g. optimizer.state.0.step), causing a RuntimeError on checkpoint load with SHARDED_STATE_DICT + fsdp_version 2.

What does this PR do?

Fixes # (issue) #3971

• Adds DefaultLoadPlanner() to dist_cp.load() in load_fsdp_optimizer, matching the DefaultSavePlanner() already used on the save side

Test plan

• Verified with FSDP2 + SHARDED_STATE_DICT config on 2x T4 GPUs

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[iavinas]

@SunMarc Could you please take a look when you have time?

I found this bug while digging into issue #3968 to better understand it. Happy to address any feedback!