[zero] Suggests a minor change to confusing variable names in the ZeRO optimizer.

[yhna940]

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📝 What does this PR do?

It seems that the variable names related to the mixed precision parameter group do not comprehensively cover its characteristics, so I suggest a few changes. These changes are very trivial, but hopefully they will alleviate some of the confusion for beginners like me.

Currently, the entire parameter group is named fp16_param_groups, and the parts managed by the gpu at the current rank are described as fp32_flat_param_groups_of_current_rank. This state perfectly represents the characteristics when the master weight is a half-tensor or the dtype specified in the __init__ method is fp16. In other cases, however, its characteristics do not correspond to the variable it. So I want it to be renamed according to the sharding state, not the data type, according to the fsdp convention of pytorch. (with names like flatten_sharded_optim_state_dict and full_optim_state_dict).

This is a related but more trivial issue, but it seems that the param_store methods don't even need to specify fp16.

Thank you :)

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