[caffe2] use 2-stage EmbeddingSpMDM interface (#32271)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32271
Use the 2-stage EmbeddingSpMDM interface in D19425982 to reduce the overhead of code cache lookup and lock contention.
Fix an issue in sparse_lengths_sum_benchmarks generating empty indices when average length is small like 1.
Test Plan: CI
Reviewed By: dskhudia
Differential Revision: D19425987
fbshipit-source-id: d5c5f0d46e0072403901809c31d516fa0f4b9b31
