[PyTorch] StorageImpl: cache size_bytes.is_symbolic() (#85309)
We've got 6 bools' worth of extra space, so let's try caching this.
Differential Revision: [D39636570](https://our.internmc.facebook.com/intern/diff/D39636570/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D39636570/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85309
Approved by: https://github.com/ezyang
