[oss][pytorch] Add quint2x4 dtype (#65545)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65545
Introduce 2bit qtensor. The new dtype added for this is c10::quint2x4
The underlying storage for this is still uint8_t, so we pack 4 2-bit values in a byte while quantizing it.
Kernels that use this dtype should be aware of the packing format. (4 2-bit values in one byte)
Test Plan: `buck test mode/dev-asan caffe2/test/:quantization -- test_qtensor`
Reviewed By: supriyar
Differential Revision: D31148141
fbshipit-source-id: 1dc1de719e097adaf93fee47c6d1b8010a3eae6c
