Add as_quantized_tensor (#20740)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20740
Provide a way to assemble quantized Tensor from int8 Tensor, scale and zero point.
Differential Revision: D15232416
fbshipit-source-id: c3a3d9d7214b1dc569214c019440c2779fbd063b
