[Core][Distributed] use device group for all broadcast #5320
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A fix to #4444
That PR was originally tested on A100, and it showed some speedup.
However, later it seems to slow down on H100 and other machines. The hypothesis is that
gloo
performs poorly while nvlink is better in these high-end machines.Before we find a good solution, we can just use device group for all the broadcast.
TODO:
investigate if it is possible and beneficial to only communicate cpu data, using mechanisms such as message queue.