[Bug][Dynamic Embedding] improper optimizier state_dict `momentum2` key while constructing PSCollection

[JacoCheung]

Describe the bug

A PSCollection should contain optimizer states besides weights. The optimizer states tensors are obtained directly from EmbeddingCollection Module.

However, the sharded_module.fused_optimizer.state_dict()['state'] does not contain key {table_name}.momentum2 because

1. TBE::get_optimizer_state() which is used by PSCollection will not return key like xxx.momentum1 or xxx.momentum2. They are customized by TBE.
2. The states keys are renamed by torchrec::EmbeddingFusedOptimizer. The first state falls back on xxx.momentum1 while the left keys are copied from above retrived results.

See the below illustration where optimizer is Adam. The expected number of state tensors should be 2, but the it eventually gives momentum1 and leaves momentum2 (which is synonymously exp_avg_sq) out.

It will pose impact on all kinds of optimizer that contains momentum2.

[iamzainhuda]

Yup this shouldn't be intended behaviour. Feel free to submit a PR I can go ahead and review.