Fuse MLA DOWN projection GEMMs

[cjld]

What does this PR do ?

add FUSE_QKV_DOWN_PROJ and FUSE_INPUT_LAYERNORM env to control the fusion.
It is ready for profiling, but still need some change to be merged.

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[maanug-nv]

is there a PR to dev that we can link? i don't see the same changes on dev branch currently?

[maanug-nv]

can fuse_input_layernorm just be an argument to this function? that seems a more consistent UX than adding environment variables for any adjustment to the spec.

[jaredcasper]

Please replace them now. We do not use environment variables to configure anything in mcore.