[bugfix] Fix signature mismatch in DDPCPUHPCAccelerator's model_to_device

[ananthsub]

What does this PR do?

model_to_device in the parent class does not take process_idx as an argument anymore. however, the child class here wasn't updated, leading to this signature mismatch. as a result, folks using the torchelastic+ddp+cpu will run into a crash in ddp_train complaining about the the process_idx argument missing

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[ananthsub]

this along with #5157 points to an opportunity for us to improve the multi-gpu/cpu CI to catch these proactively. @tchaton @Borda what do you think we could set up for these use cases?

[codecov]

Codecov Report

Merging #5505 (fb0c300) into master (7f352cb) will not change coverage.
The diff coverage is n/a.

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  Coverage      93%     93%           
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  Files         135     135           
  Lines       10007   10007           
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  Hits         9341    9341           
  Misses        666     666