Added more descriptive error message for index out of range (#21758)
Summary:
https://github.com/pytorch/pytorch/issues/21535
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21758
Differential Revision: D15922915
Pulled By: Chillee
fbshipit-source-id: dcb301a661c359f27869200ee241ec272ef50d3a
