Support PyTorch reduce_all in DDP using SMDDP so we can Parallelize Validation

Please support reduce_all using the Data Parallel DDP API for tensors while using the SMDPP backend so that we can run our validation runs in parallel rather than on a single rank.

`validation_loss = dist.all_reduce(loss, op=ReduceOp.AVG)`

Results in:

`RuntimeError: SMDDP does not support: ReduceOp`

For example, so we don't have to restrict validation to a single rank like this:

https://github.com/aws/amazon-sagemaker-examples/blob/c266495f4a4b8e9c65f288edad5f0729c5ca3959/training/distributed_training/pytorch/data_parallel/mnist/code/train_pytorch_smdataparallel_mnist.py#L255