Implement gather primitive for ProcessGroupNCCL (#66745)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66745
This PR implement NCCL gather and add gather to ProcessGroupNCCL using nccl send/recv api.
NCCL doesn’t directly provide primitives for gather, so we need to be implemented on top of NCCL’s send/recv API.
1. In ProcessGroupNCCL.cpp, the outputTensors are first flattened, then inputTensors and outputFlattened are passed by the collective class to gather() function in nccl.cpp.
1. In nccl.cpp, gather is implemented using ncclSend/ncclRecv: all the ranks send inputTensor to the root rank, and the root rank uses a for loop to receive these inputTensors.
ghstack-source-id: 147754838
Test Plan:
test_gather_ops
test_gather_checks
test_gather_stress
Reviewed By: pritamdamania87
Differential Revision: D29616361
fbshipit-source-id: b500d9b8e67113194c5cc6575fb0e5d806dc7782
(cherry picked from commit d560ee732eb559782a2d1d88b3cf118dcfc404bc)
