Add switch-channel NVLS cross-rank barrier

[RJ Souza (Empyreus)]

Adds a device-side cross-rank barrier built on NVLS (multimem) atomics

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[Copilot]

Pull request overview

This PR adds a device-side, cross-rank barrier for NVLS (multimem) SwitchChannels and wires it through the executor and Python DSL so kernels can synchronize all ranks in an NVLS group without an O(n²) semaphore mesh.

Changes:

• Add NVLS-backed barrier state to NvlsConnection/SwitchChannel and expose it via SwitchChannelDeviceHandle::barrier().
• Introduce a new executor op (MULTI_BARRIER / "gbarrier") with device-side implementation that grid-syncs then issues a single multimem arrival per rank.
• Add Python DSL support (SwitchChannel.barrier(...)) plus a single-node allgather example that uses the new barrier.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/core/switch_channel.cc	Creates an auxiliary NVLS multicast for barrier state and stamps barrier pointers onto bound SwitchChannels.
src/core/include/execution_kernel.hpp	Implements MULTI_BARRIER device handler (grid-converge → one multimem barrier → release).
src/core/include/execution_common.hpp	Adds OperationType::MULTI_BARRIER.
src/core/executor/execution_plan.cc	Parses "gbarrier" and maps JSON switch_channel_id into Operation.nvlsInputIndex.
include/mscclpp/switch_channel.hpp	Stores barrier resources on NvlsConnection and propagates them into SwitchChannel handles.
include/mscclpp/switch_channel_device.hpp	Adds SwitchChannelDeviceHandle::barrier() API and barrier metadata fields.
python/mscclpp/language/internal/types.py	Adds DSL instruction enum value for "gbarrier".
python/mscclpp/language/internal/operations.py	Adds GroupBarrier operation with instancing semantics collapsed onto one per-rank syncer.
python/mscclpp/language/internal/buffer_access.py	Treats group_barrier as an access-sequence breaker like other barriers/nops.
python/mscclpp/language/channel.py	Adds SwitchChannel.barrier() plus rank-view convenience wrapper and docstrings.
python/mscclpp/language/tests/single_node/allgather_nvls_zero_copy_barrier.py	Example program exercising the new switch-native barrier.

[Changho Hwang (chhwang)]

LGTM