[functorch] Only generate vmap plumbing for operations in the allowlist (pytorch/functorch#758)
After this change, everytime someone adds a batching rule using
VMAP_SUPPORT, then that person needs to add the batching rule to the
allowlist.
The motivation behind this change is to keep VmapGeneratedPlumbing.h as
short as possible so that functorch can build with more pytorch/pytorch
commits.
We can delete the allowlist after torchgen goes in.
