[core][distributed] add pynccl broadcast

[youkaichao]

No description provided.

[github-actions]

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[tlrmchlsmth]

looks good

I noticed that many of the tests in test_pynncl are checking the mean when it looks like they ought to be all the same -- we can fix it up in a follow-up

[youkaichao]

many of the tests in test_pynncl are checking the mean when it looks like they ought to be all the same -- we can fix it up in a follow-up

agree. this is just historical code. welcome to update it.