Sparse CSR: Handle zero matrix consistently for triangular_solve (#71304)
Summary:
This PR enables `test_block_triangular` tests on the CPU.
These tests revealed that there was a problem with how the nnz==0 case is handled. Now we return a tensor filled with NaNs both on CUDA and CPU.
cc nikitaved pearu cpuhrsch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71304
Reviewed By: davidberard98
Differential Revision: D33600482
Pulled By: cpuhrsch
fbshipit-source-id: d09cb619f8b6e54b9f07eb16765ad1c183c42487
