[distributed] remove pynccl's redundant stream

[cennn]

I've streamlined the PyNcclCommunicator codebase by removing redundant stream initializations and assignments.

[github-actions]

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[youkaichao]

thanks for the great work!

[youkaichao]

also cc @tlrmchlsmth

[youkaichao]

merge as buildkite/fastcheck/pr/entrypoints-test succeeds.

[gshtras]

This (and to an extent subsequent #11749) adds a significant performance regression on ROCm:
python benchmarks/benchmark_latency.py --enable-chunked-prefill False --load-format dummy --dtype float16 --num-iters-warmup 2 --num-iters 5 --batch-size 1 --num-scheduler-steps 10 --input-len 2048 --output-len 128 --model meta-llama/Llama-2-70b-chat-hf -tp 8
drops from 1.5s to 1.7s

[youkaichao]

@gshtras this is very surprising, I thought this PR changes nothing.

Upon reflection, it seems it might be related with torch.cuda.current_stream() , I measured the cost to be around 3us, and it can accumulate in eager mode (e.g. prefill without cudagraph).

Can you help measure the cost of torch.cuda.current_stream() on ROCm?