Support sparse.sum on empty sparse tensor (#71091)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71091
Fixes https://github.com/pytorch/pytorch/issues/65394
The masked sum on a full input tensor (of any layout) with an all-true mask is the same as the sum on the strided input tensor (after applying `to_dense` to sparse inputs).
Since masked sum uses `torch.sparse.sum` then, for the simplicity of masked reductions implementations, its reduction behavior ought to be defined by the behavior of the `torch.sum`. This PR implements the behavioral connection with respect to the directional summation of empty sparse tensors that correspond to all-zero strided tensors.
cc nikitaved pearu cpuhrsch
Test Plan: Imported from OSS
Reviewed By: davidberard98
Differential Revision: D33651750
Pulled By: cpuhrsch
fbshipit-source-id: 703891bff88c8da6270b4272f5d2da81688db67d
(cherry picked from commit 53f97e80f7520594e9977ad61a1a727dadade645)
