Clean up Storage/StorageImpl constructors (#16948)
Summary:
Small cleanup while doing https://github.com/pytorch/pytorch/pull/16857:
- rename C2 constructors as create_legacy
- remove duplicated constructors
- make resizable flag non-default
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16948
Differential Revision: D14062755
Pulled By: dzhulgakov
fbshipit-source-id: 3b7b4ec9cdf67d2628cccc001156e040006b673e
