[frontend] allow var_mean to be implemented in one pass

[peterbell10]

Currently PyTorch inductor is forced to implement torch.var_mean as two passes over the input data, which causes a slowdown for batch norm. To allow single-pass computation we need a new reduction operator tl.welford(mean, m2, count) which implements the combination step of parallel Welford's algortihm.

A more general solution might be to instead add a tl.reduce which takes a function acting on scalars, so users can write their own reductions without needing to change the triton language.

[ptillet]

I feel like such a function would not belong in the triton.language namespace. As you suggested, the right solution is probably to have some more flexible reduce op, but this would be a significant amount of work as none of our optimizer and backend codegen is ready to support instructions that take in functions as arguments