…reduce (#137474)

## This Stack

Implement custom all-reduce algos available in `IntraNodeComm` as `symm_mem` ops and replace the existing `IntraNodeComm` kernels with them.

## This PR

Implement `symm_mem::multimem_one_shot_all_reduce_out`. The out-variant is more suitable for `IntraNodeComm` integration.
Pull Request resolved: #137474
Approved by: https://github.com/Chillee
ghstack dependencies: #137471, #137472, #137473

…m ops (#137475)

## This Stack

Implement custom all-reduce algos available in `IntraNodeComm` as `symm_mem` ops and replace the existing `IntraNodeComm` kernels with them.

## This PR
- Replaces one-shot all-reduce with `symm_mem::one_shot_all_reduce_out`
- Replaces two-shot all-reduce with `symm_mem::two_shot_all_reduce_`
- Removes HCM all-reduce (at least for now). Due to the nature of its accumulation order, we can't guarantee the numerical consistency across all ranks.
- Removes the `IntraNodeComm` python binding (its original purpose is superceded by `SymmetricMemory`).
- Removes methods that were made for the python binding.
- Replaces nvlink detection logic with `DMAConnectivityDetector`.

Pull Request resolved: #137475
Approved by: https://github.com/Chillee
ghstack dependencies: #137471, #137472, #137473, #137474

…ating bfloat16 with multimem.ld_reduce (#137529)

This provides better accuracy without additional cost.

Also added documentation to `multimem_one_shot_all_reduce` to note the numerical caveats.
Pull Request resolved: #137529
Approved by: https://github.com/Chillee
ghstack dependencies: #137471, #137472, #137473, #137474, #137475

- Previously the detection would fail before user calling APIs such as `torch.cuda.set_device()`. This is because the detection logic requires nvml initialization. In this PR, we added explicit nvml initialization (which idempotent).
- Previously any nvml issue occurred in the detection logic would result in fatal error. Now we issue an informative warning and return a topology assuming no NVLink connectivity.

Pull Request resolved: #137530
Approved by: https://github.com/Chillee
ghstack dependencies: #137471, #137472, #137473, #137474, #137475, #137529

ghstack-source-id: 3e66c10
Pull Request resolved: pytorch/pytorch#137473