Perform appropriate CUDA stream synchronization in distributed autograd.
The local autograd engine performs appropriate stream synchronization
between autograd nodes in the graph to ensure a consumer's stream is
synchronized with the producer's stream before executing the consumer.
However in case of distributed autograd, the SendRpcBackward function receives
gradients over the wire and TensorPipe uses its own pool of streams for this
purpose. As a result, the tensors are received on TensorPipe's stream pool but
SendRpcBackward runs on a different stream during the backward pass and there
is no logic to synchronize these streams.
To fix this, I've enhanced DistEngine to synchronize these streams
appropriately when it receives grads over the wire.
Differential Revision: [D27025307](https://our.internmc.facebook.com/intern/diff/D27025307/)
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