Add defined() to caffe2::Tensor (#16125)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16125
Add defined() method to check whether the Tensor is defined.
Reviewed By: ezyang
Differential Revision: D13719222
fbshipit-source-id: ff8efef2159ed1026bd16acaea40c768a1e20a47
