Add IB merge auto view selection

[CyberSecurityErial]

Description

Draft implementation for #2216.

This PR adds an opt-in auto-selection mode for NCCL_IB_MERGE_NICS on two-node
IB/RoCE systems. The default behavior remains unchanged:

NCCL_IB_MERGE_NICS=0  # force unmerged
NCCL_IB_MERGE_NICS=1  # current default merged behavior
NCCL_IB_MERGE_NICS=2  # new opt-in auto mode

When auto mode is enabled, NCCL builds two topology/channel candidates before
normal transport setup:

• UNMERGED
• MERGED_DEFAULT

It gathers each candidate's searched ring channel count across ranks, selects
the view with the larger global minimum channel count, then rebuilds the
official topology using only the selected view before the normal graph search.

This is intentionally a simple first version. It does not run a benchmark, does
not establish duplicate transport connections, and does not do per-channel or
per-edge mixed merge selection.

Related Issues

Fixes #2216

Changes & Impact

• Add a NET merge-view topology entry point.
• Make IB virtual NIC creation idempotent so repeated candidate construction can
  reuse existing vNICs instead of appending duplicates.
• Add a MergeAuto candidate builder for unmerged and default merged topology
  views.
• Select the official view using only globalMinChannels.
• Rebuild the official topology with the selected view before normal graph
  search.
• Keep unsupported cases on the existing default path.

Current first-version scope:

• two-node communicators only
• IB/RoCE NET transport only
• init-time selection only

Out of scope for this PR:

• multi-node auto selection
• runtime benchmark feedback
• rail/bandwidth scoring
• per-collective switching
• channel-level or edge-level mixed merge

Performance Impact

Default behavior is unchanged unless NCCL_IB_MERGE_NICS=2 is explicitly set.

For NCCL_IB_MERGE_NICS=2, communicator initialization performs two extra
candidate topology/channel searches before selecting the official view. The data
path uses the selected normal topology only.

Validation performed:

• Built and tested in a two-node multi-HCA A800/RoCE environment.
• Compared manual NCCL_IB_MERGE_NICS=0, manual NCCL_IB_MERGE_NICS=1, and
  auto NCCL_IB_MERGE_NICS=2.
• In the tested case, auto selected the unmerged view, matching the better manual
  setting.
• Rebased onto current upstream master and retested successfully in the same
  environment.