[Kernel] Enable BFloat16 inputs in fused Marlin MoE kernels

[ElizaWszola]

This enables BFloat16 input type in the kernels, so we don't have to perform the type conversion hack in GPTQ codepath.

[github-actions]

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[ElizaWszola]

Closing this for now. Converting to bf16 on the fly doesn't seem to impact the throughput and actually results in lower latency in some cases (as measured with sonnet benchmark).