Don't segfault on trying to get data_ptr of sparse tensor. (#18347)
Summary:
Also asserts in storage_initialized that there is a storage.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18347
Differential Revision: D14582028
Pulled By: gchanan
fbshipit-source-id: df3f5d181188f39e361839169fd054539c3b2839
