[quant] Backend string for the quantized types (#49965)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49965
Without this checking the type of the quantized tensor using `type` would throw an error.
After this PR running the `type(qx)`, where `qx` is a quantized tensor would show something like `torch.quantized.QUInt8`.
Test Plan: Not needed -- this is just a string description for the quantized tensors
Differential Revision: D25731594
Reviewed By: ezyang
Pulled By: z-a-f
fbshipit-source-id: 942fdf89a1c50895249989c7203f2e7cc00df4c6
