introduce TensorBase::mutable_data_ptr<T> (#97874)
See D44409928 for motivation.
Note that we keep the const-ness of the existing data_ptr<T>() member so
that we don't have to change all references atomically. We just change
the ones here that we have higher confidence with.
Differential Revision: [D44497685](https://our.internmc.facebook.com/intern/diff/D44497685/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D44497685/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97874
Approved by: https://github.com/ezyang
