ciflow/trunk/186163: Require NCCL >= 2.27 and drop version gates for older NCCL

Raise the compile-time minimum NCCL version to 2.27 and remove the

preprocessor and runtime version gates that guarded features introduced in

NCCL 2.27 or earlier. The introduction version of every gated feature was

cross-checked against the NCCL release notes before its gate was removed:

AVG (2.10), PreMulSum (2.11), ncclGetLastError/ncclRemoteError (2.13),

non-blocking comms + ncclConfig_t (2.14), ncclCommSplit (2.18),

ncclCommRegister/ncclMemAlloc (2.19), group abort (2.22),

ncclCommInitRankScalable (2.23), FP8 e5m2/e4m3 (2.24), QoS/trafficClass

(2.26), and ncclCommShrink / ncclCommWindowRegister / symmetric memory /

collnetEnable / CTAPolicy / nvlsCTAs (2.27).

Suggested review order:

1. NCCLUtils.hpp: the static_assert is bumped from 2.7 to 2.27 and the
   
   now-unconditional feature macros are deleted. This is the source of
   
   truth that makes every gate below dead code.


2. NCCLUtils.cpp, ProcessGroupNCCL.{hpp,cpp}, init.cpp: the dead
   
   #ifdef/#ifndef/#else branches for those macros are removed and the
   
   live branch inlined; the runtime getNcclVersionNumber() >= 2.22
   
   (group abort) and version < 2.19 (mem allocator) checks are dropped.


3. cuda/nccl.{h,cpp}: the local 2.13/2.14 feature macros and the ancient
   
   NCCL 1.x / 2.0 / 2.7 / 2.12.10 guards are removed; only NCCL-presence
   
   gates (USE_NCCL, defined(NCCL_MAJOR)) and >= 2.28 gates remain.


4. symm_mem/nccl_dev_cap.hpp and the C++ test: NCCL_HAS_SYMMEM_SUPPORT is
   
   made unconditional within USE_NCCL.

Gates for versions newer than 2.27 are kept as-is (>= 2.28 ncclAlltoAll,

NCCL_HAS_SYMMEM_DEVICE_SUPPORT, NCCL_HAS_COMM_OFFLOAD 2.29.7,

NCCL_HAS_MAX_P2P_PEERS 2.30, etc.), as is the defensive runtime

version >= 2.27 check inside ProcessGroupNCCL::shrink().

Two macros were not deleted outright because they encode NCCL availability,

not just a version, and are evaluated in non-NCCL build configurations:

• HAS_NCCL_BF16_DATATYPE (nccl.h) keeps its CUDA-bf16/ROCm structure; only
  
  the NCCL_MINOR >= 10 subexpression is dropped (now NCCL_MAJOR >= 2),
  
  preserving the "NCCL absent => disabled" behavior its consumers rely on.


• NCCL_HAS_SYMMEM_SUPPORT is kept as a macro but defined unconditionally
  
  inside #if USE_NCCL, since the symmetric-memory source files use it to
  
  gate on NCCL presence as well as version.

Test Plan:

Build against NCCL 2.29.7 (satisfies the new static_assert):

uv pip install -e . --no-build-isolation -v

Lint the changed files:

lintrunner -a torch/csrc/distributed/c10d/NCCLUtils.hpp \

  torch/csrc/distributed/c10d/NCCLUtils.cpp \

  torch/csrc/distributed/c10d/ProcessGroupNCCL.cpp \

  torch/csrc/distributed/c10d/ProcessGroupNCCL.hpp \

  torch/csrc/distributed/c10d/init.cpp \

  torch/csrc/cuda/nccl.cpp torch/csrc/cuda/nccl.h \

  torch/csrc/distributed/c10d/symm_mem/nccl_dev_cap.hpp \

  test/cpp/c10d/ProcessGroupNCCLTest.cpp

This PR was authored with the assistance of Claude, an AI coding assistant.