Add triangular solve function for sparse CSR tensors in XPU

[tszulist-hbn]

Resolves: #3167

This PR adds the SparseCsrXPU dispatch for triangular_solve by converting the sparse matrix to dense and delegating to the existing dense XPU triangular_solve implementation. This follows the same pattern used by other XPU sparse ops (addmm, baddbmm, etc.).

Changes:

yaml/native/native_functions.yaml — Register SparseCsrXPU: triangular_solve_out_sparse_csr_xpu dispatch for both triangular_solve.X (structured out variant) and triangular_solve (structured delegate).
src/ATen/native/sparse/xpu/SparseCsrTensorMath.cpp — Implement triangular_solve_out_sparse_csr_xpu() which handles the zero-nnz edge case (fills X with NaN, matching CUDA behavior) and otherwise converts sparse A to dense before calling at::triangular_solve.

Tests verified:

test_block_triangular_solve — 64/64 variants pass (block_size 2/3, int32/int64, contiguous/noncontiguous, float32/float64/complex64/complex128)
test_sparse_triangular_solve_xpu — 4/4 variants pass (float32/float64/complex64/complex128)

[Copilot]

Pull request overview

Adds SparseCsrXPU support for triangular_solve by wiring a SparseCsrXPU dispatch and implementing an XPU sparse CSR fallback that densifies A and calls the existing dense XPU solve.

Changes:

• Register SparseCsrXPU dispatch for triangular_solve.X (and delegate triangular_solve to it).
• Implement triangular_solve_out_sparse_csr_xpu in the XPU sparse CSR math file using A.to_dense() + at::triangular_solve.
• Add an _nnz()==0 fast-path intended to match CUDA behavior (fills X with NaNs).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
yaml/native/native_functions.yaml	Adds SparseCsrXPU dispatch registration for triangular_solve.X and structured delegate entry for triangular_solve.
src/ATen/native/sparse/xpu/SparseCsrTensorMath.cpp	Implements SparseCsrXPU out-kernel that densifies A and delegates to dense at::triangular_solve, with an _nnz()==0 special-case.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[guangyey]

Overall LGTM. Could you please add a test case? Maybe test/regressions is a good place.
Defer to @CuiYifeng