Function fp4_fallback 

pub fn fp4_fallback(device: &Device) -> Exec

2026-10-01: How device runs an NVFP4 activation without an FP4 block-scale kernel: on the FP8 MMA through the exact conversion where it has one, else no path; Exec::NoFp4Kernel when the device has the FP4 MMA and only the kernel is missing.