Update allgather fallback algo

[Binyang2014]

Enhancements to all-gather operation, a temporary solution to fix the memory overhead when integrating msccl++ with pytorch.
This solution will not register input/output buffer to msccl++, so the temp output buffer for allgather could be reused by torch automatically.

• Introduced a new allgather8 kernel function in apps/nccl/src/allgather.hpp to handle larger data sizes more efficiently. This includes double buffering to hide synchronization overhead and support for both in-place and out-of-place operations.
• Modified the allgather function to decide between allgather6 and allgather8 based on data size and platform, improving performance for large data sizes.

Configuration and environment improvements:

• Added a new environment variable MSCCLPP_DISABLE_CHANNEL_CACHE to control whether the channel cache is disabled, enhancing configurability. This variable is now part of the Env class and is logged during environment initialization.
• Removed the redundant global variable mscclppDisableChannelCache from src/debug.cc and updated its usage to refer to the new environment variable.

[caiomcbr]

LGTM

[Binyang2014]

/azp run

[azure-pipelines]

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