[2/N][ST deprecate][BE] Remove Replicate Tensor convert from DDP and PTD (#95450)
No use is found for this ST/Replicated Tensor based DDP. As part of ShardedTensor migration, let's remove this logic. Trying to undo everything in https://github.com/pytorch/pytorch/pull/75753.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95450
Approved by: https://github.com/wanchaol
