Ring-based decomposition for Allgather+GEMM overlap ATen implementation

[nsarka]

Implementation using ATen of https://docs.google.com/document/d/1Fzr9Zs2Dqfj3e4yR8LKxFrRqC1EkMUfQczJMYQGJQUI/edit?tab=t.0#heading=h.5x7hptdjzhet