DDP forward support custom stream accelerated copy. (#98723)
At present, DDP forward uses `_get_stream` to get a stream,which is cudaStream.
If the custom module already registered to torch, I can use `getattr` to get it and it's stream. Then, the custom stream is used to copy the tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98723
Approved by: https://github.com/ezyang
