I found that returning the copy is actually useful in situations where you might do something like:

```
ret = _copy_state_dict(obj, cache)
ret.update(some_other_values)
```

and would like `cache` not to change structure from `ret.update(some_other_values)`.  Open to some notes here, not returning a copy might force the user to do some additional copies for this case.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/123567
Approved by: https://github.com/wz337