Scratch buffer copy-based implementation of ncclBcast / ncclBroadcast

[SreevatsaAnantharamu]

Encountered a hang with rccl-test's ncclBcast runs. In rccl-test with ncclBcast, the buffer changes only for the root-gpu b/w test runs. So, the channel key changes only for the root gpu and remains the same for the other gpus. So, the root gpu starts to create a new sm channel while the other gpus do not create new sm channels. Hence, a hang occurs.

To fix this, we have to use copy-based implementation. This is the simplest copy-based implementation. Uses a static channel key with sm channels created using scratch buffer as the remote memories.

[SreevatsaAnantharamu]

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