Add reduce_scatter_tensor in place of _reduce_scatter_base (#85867)
This is a twin PR similar to the one for `all_gather_into_tensor` (#85686).
The philosophy for renaming `_reduce_scatter_base` instead of merging it is described in #85686.
Cc @rohan-varma @H-Huang @crcrpar @ptrblck @mrshenli
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85867
Approved by: https://github.com/crcrpar, https://github.com/H-Huang
