Add set_quantizer_ for QTensor (#21852)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21852
To enable change of q_scale and q_zero_point in `copy_`
Differential Revision: D15793427
fbshipit-source-id: a7040b5b956d161fd6af6176287f4a4aa877c9be
