[inductor] Add ir.WelfordReduction with multiple outputs (#104725)
This replaces `var_unnormalized` reduction type with `welford_reduce` which takes the input data and outputs not just the variance, but also the mean and weights which account for the full welford accumulator state. Thus we can avoid re-computing the mean, and we now have enough information to create a multilayer reduction which I implement here by adding a second reduction type called `welford_combine` which reduces over all three inputs simultaneously.
Multi-layer support is particularly important as normalization operators like BatchNorm are being split in many timm models, which meant `var_unnormalized` had to fall back to two-pass variance calculation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104725
Approved by: https://github.com/lezcano
