Add clone() implementation for QTensor (#22510)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22510
Added a new function to implement clone operation on quantized tensors. Also added a test case which can be tested as shown in test plan.
This change is required to be able to call torch.jit.trace on quantized models.
Clone implementation calls copy_ on QTensor internally.
Differential Revision: D16059576
fbshipit-source-id: 226918cd475521b664ed72ee336a3da8212ddcdc
