[mxfp8 moe training] update readme and tests

[danielvegamyhre]

• Update readme to use _to_mxfp8_then_scaled_grouped_mm, the more explicit convenience function
• Fix individual primitive benchmark latest benchmarks for this primitive (previously I accidentally included MoE layer speedup instead of individual primitive speedup in this section)
• Update tests to use latest torchtitan refactor

[pytorch-bot]

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