[Quant][core][refactorization] Refactored qlinear_unpack.cpp into an implementation file and higher level call registration and definition file (#73956)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73956
This refactorization is analogous to the one done in https://github.com/pytorch/pytorch/pull/73773, but for linear instead of conv.
Refer to the summary in that PR for details.
Test Plan:
```
python test/test_quantization.py
```
Imported from OSS
Differential Revision:
D34824249
D34824249
Reviewed By: ezyang
Pulled By: dzdang
fbshipit-source-id: 5d33c16dde7f9c7606a26802855094bea4757867
(cherry picked from commit b1ab945f9504e834481528172be5f24c7c163fee)
