[caffe2] Enable copying for caffe2::Tensor (#36468)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36468
Since `caffe2::Tensor` is now refcounted, enabling copy constructor and the copy assignment operator should be fine.
Test Plan:
```
buck test mode/dev //caffe2/caffe2:caffe2_test_cpu -- TensorTest
```
AI/AF canaries with changes up to D20959214:
https://our.intern.facebook.com/intern/experiment_store/experiment/3298538636995/#commit1-commit2
https://our.intern.facebook.com/intern/experiment_store/experiment/2199027015376/#commit1-commit2
AI/AF canaries on this diff:
https://our.intern.facebook.com/intern/ads/canary/425960191574068914/
https://our.intern.facebook.com/intern/ads/canary/425960179835413033/
Reviewed By: yinghai
Differential Revision: D20985924
fbshipit-source-id: ead5f5ceff23d0adc06d598128de16a5533d767b
