Fix FSDP CPU offloading when shards are already on CPU

[zhixiangli]

What does this PR do?

Fixes #21804

Offloading to CPU when FSDP is on CPU triggers a use-after-free in PyTorch's FlatParamHandle.to_cpu(). This change checks if the parameters are on CPU and disables offload_to_cpu if they are.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (4819088) to head (e00c8ea).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21805   +/-   ##
=======================================
  Coverage      87%      87%           
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  Files         270      270           
  Lines       24010    24012    +2     
=======================================
+ Hits        20787    20792    +5     
+ Misses       3223     3220    -3