Port FBGEMM MoE kernels to Windows

[philpax]

We use FBGEMM MoE kernels to optimise inference; however, these kernels are not natively available on Windows (they are only built for manylinux). As a workaround, we are disabling FBGEMM for Windows in #22, but we would prefer to have native ports of these kernels to ensure that performance has parity between the OSes.

[philpax]

I've put this down in the milestone, but it's not a hard blocker - nonetheless, we should still do it if at all possible.