Mismatch with opt_einsum on FLOPs counting

[mofeing]

I'm not sure why there is this mismatch but I've found the number of FLOPs reported by EinExprs is approximately half of what opt_einsum reports.

I believe that it might be because we are only counting floating-point multiplications, and not sums.

If that is the case, should we really consider counting sums? Current implementations use fmad instructions (i.e. Fused Multiply And Add) which fuse multiplication and addition in only one instruction.