Kernel Numerics Verifier

Verify a handwritten kernel with three implementations:

• golden: eager FP64 implementation. Use FP32 only when FP64 is unsupported.
• reference: eager implementation in the kernel's input and output dtype.
• target: handwritten Triton or CUDA kernel in the same dtype as reference.

Generate low-precision inputs once, then cast those exact values to FP64 for golden. This measures kernel arithmetic error without mixing in input quantization error.

Correctness gate

For each output tensor and gradient tensor, compute in the golden dtype: