cuda.parallel: Should stream be a required parameter?

[shwina]

In #3348 (comment), it came up that we may want to consider making stream a required argument wherever a CUDA stream can be passed.

At the moment, stream is not a required parameter and defaults to None (translating to nullptr or "default stream" at the C++ layer). This aligns with underlying C++ APIs which also use stream=0 as a default (example).

However, as pointed out in the discussion above, users can forget to pass a stream in situations like the following:

with stream as s:
    arr = cp.random.random(...)

reduce_into(..., d_in=arr, ...)  # forgot to pass s here for whatever reason

This is seemingly a broader problem in the ecosystem (any API from any library accepting a CUDA stream has the same problem). So it would be good to hear perspectives from e.g., @vyasr as I know cuDF is also considering exposing streams in their APIs.