Update on "Fix CUDA RPC Stream Synchronization"

When converting RPC Message into Python objects, we were not using
a CUDAFuture for the chained Future. As a result, the streams are
not synchronized when calling `rpc_async(...).wait()`. This commit
uses `Future::then` API to create the chained Future, which will
be creating a CUDAFuture if the existing Future is a CUDA one.

fixes #50881
fixes #50839

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