use _sparse_coo_tensor_unsafe in coalesce for speedup (#21214)
Summary:
Studied why sparse tensor coalesce was slow: issue #10757.
Using nv-prof, and writing a simple benchmark, I determined bulk of the time was used ``kernelTransformReduceInnermostDimIndex``, which is called when sparse tensor is constructed with sparse_coo_tensor when it does sanity check on the minimum and maximum indices. However, we do not need this sanity check because after coalescing the tensor, these min/maxs won't change.
On my benchmark with 1 million non-zeros, the runtime of coalesce. was about 10x from 0.52s to 0.005 sec.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21214
Reviewed By: bddppq
Differential Revision: D15584338
Pulled By: akyrola
fbshipit-source-id: a08378baa018dbd0b45d7aba661fc9aefd3791e0
