cuda: optimize SOLVE_TRI using registers and FMAF (llama/17703)

cuda: optimize SOLVE_TRI using registers and FMAF (llama/17703)

* ggml-cuda: optimize solve_tri_f32_fast and fix stride handling

- Switch from using shared memory for the RHS/solution matrix to a register-based approach (x_low, x_high), reducing shared memory pressure and bank conflicts.
- Implement explicit `fmaf` instructions for the reduction loop.
- Update kernel arguments to pass strides in bytes rather than elements to align with standard ggml tensor arithmetic (casting to `char *` before addition).
- Remove unused `MAX_K_FAST` definition.

* Small cleanup

* Remove comments in solve_tri.cu

* Update ggml/src/ggml-cuda/solve_tri.cu

Co-authored-by: Johannes Gäßler <redacted>
* Update ggml/src/ggml-cuda/solve_tri.cu

Co-authored-by: Johannes Gäßler <redacted>
* Update ggml/src/ggml-cuda/solve_tri.cu

Co-authored-by: Johannes Gäßler <redacted>
* Use const for variables in solve_tri.cu

* Replace fmaf with more readable code

* remove last fmaf

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Co-authored-by: Johannes Gäßler <redacted>