[pallas:triton] Emit a better error message for matmul with non-2D operands
Triton seems to support both 2D and 3D operands now, the latter case being a
batched matmul. We need more changes in the lowering to support 3D, so I will
leave it out of scope here.
Fixes #26013.
PiperOrigin-RevId: 733293299