Add qint8 type (int8_t) (#19984)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19984
Add qint8 for QTensor, with underlying type of int8_t
Reviewed By: jianyuh
Differential Revision: D15150715
fbshipit-source-id: 57580f599d46f9323af5ce462dbbc464b25e40d7
