Add MXFP casting kernels from triton Repro

[drisspg]

Summary

They have recently published alot of good upcast and downcast kernels: https://github.com/triton-lang/triton/blob/main/python/triton_kernels/triton_kernels/numerics_details/mxfp.py We should update the ones we have in AO and bench against Inductor