[pytorch] Relax the check that makes sure number of outs equals number of returns (#76049)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76049
## Context
We are trying to add an out variant for an existing operator, e.g.,:
```
chunk.out(Tensor self, int chunks, int dim=0, *, Tensor(a!)[] out) -> Tensor(a!)[]
```
Notice the out argument is a mutable list of tensors. The existing guideline defined in [model.py](https://fburl.com/nn299ifx) requires the same argument type to be returned from this operator. Given the fact that we don't support mutable tensor list as a return type and it seems not useful to add such a return type.
The solution I'm proposing is to relax the constraint that the number of outs needs to be the same as the number of returns, so we can return a `void`.
```
chunk.out(Tensor self, int chunks, int dim=0, *, Tensor(a!)[] out) -> ()
```
Test Plan: Rely on existing CI
Reviewed By: ezyang, iseeyuan
Differential Revision: D35737310
fbshipit-source-id: 66b5738cc1dcd13d532a6c97fea979bd58f381df
(cherry picked from commit 9aac5493285cd4f49a07053edfa5916c449a930c)
