Route Tensor::matmul and matmul_backward through cuBLAS Sgemm instead of
the hand-written tiled kernel. fp32 → same GEMM up to rounding order, so
the T3 cuBLAS tolerance and downstream grad-checks are preserved.
- cublas.rs: thread-local persistent handle + row-major sgemm helper with
transpose flags (col-major⟺row-major as the T3 oracle does).
- matmul_backward: dA/dB via cuBLAS OP_T, dropping the two transpose
kernels + their allocations the T3 version ran.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Gahow Wang
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