[generate_vmap_rule] reductify_leaf helper function (#90965)
As seen in
https://docs.google.com/document/d/1bIQkWXy3J35_20c_a5kchikabBW5M8_uRAhl0BIMwU4/edit
`reductify_leaf(grad_input, ...)` is a helper function that processes a
single grad_input Tensor. The reason why we need it is:
- the grad_input has some optional bdim
- the input has some optional bdim
- if these are different, we need to coerce the grad_input into having
the same shape as the input, either by reducing or expanding the
grad_input.
Note that there is a special case in autograd that the user is allowed
to return a grad_input Tensor that is an expanded version of the
original input tensor. In this case, autograd automatically reduces
grad_input to the same shape as the input. Unfortunately this logic
doesn't work when bdims are involved, so we manually handle it in
`reductify_leaf`.
Test Plan:
- tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90965
Approved by: https://github.com/soulitzer
