distinguish implementation of data() and mutable_data() on TensorImpl (#98732)
The old style had them both going through a mutable method on Storage,
which would prevent us from implementing checks differently depending
on whether we are writing or reading.
Differential Revision: [D44831044](https://our.internmc.facebook.com/intern/diff/D44831044/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98732
Approved by: https://github.com/ezyang
