PR #11422: [NVIDIA GPU] Remove control knobs for each individual async collective and use the global xla_gpu_disable_async_collectives
PR #11422: [NVIDIA GPU] Remove control knobs for each individual async collective and use the global xla_gpu_disable_async_collectives

Imported from GitHub PR https://github.com/openxla/xla/pull/11422

Currently we have 1 global flag(xla_gpu_enable_async_collectives) to control whether we want to asynchronize collectives or not. This flag overrides all other control knobs for each collective. The usage of it is confusing, instead we introduce a new flag xla_gpu_disable_async_collectives which will consolidate all the async flags we have now. We remove xla_gpu_enable_async_collectives and all other individual control knobs.
Sample usage:
xla_gpu_disable_async_collectives=allreduce,reducescatter
disables async allreduce and reducescatter
By default it's empty which indicates enabling async for all collectives.

Copybara import of the project:

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afff139cb742662801c49052b70a8f234bb280e4 by TJ Xu <tjx@nvidia.com>:

Remove control knobs for each individual async collective and use the
global xla_gpu_enable_async_collectives

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e1385c5b1f82aee1d3ea98e5d453c3e60fc5fa8a by TJ Xu <tjx@nvidia.com>:

Consolidate all flags into one

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72020bef1368f514760cbfd6630c22d2348121c9 by TJ Xu <tjx@nvidia.com>:

Change description of xla_gpu_disable_async_collectives

Merging this change closes #11422

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11422 from Tixxx:tixxx/flag_clean_up 72020bef1368f514760cbfd6630c22d2348121c9
