Clean up some sparse code. (#18962)
Summary:
1) sparse_dispatches in native_parse was not used anymore, got rid of it.
2) got rid of overloaded sizes_ in SparseTensorImpl, which just uses the base implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18962
Differential Revision: D14811545
Pulled By: gchanan
fbshipit-source-id: 2fa60ef50456b5f605caa63beae1d8d2542fd527
