Update `std_mean/var_mean/nanmean/nansum` signatures with `int[1]? dim` (#82912)
### Description
Change the type of the `dim` arg for `std_mean/var_mean/nanmean/nansum` to `int[1]?` in `native_functions.yaml`
### Issue
Part of #29137
### Testing
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82912
Approved by: https://github.com/albanD
