Add common_quantized test case utilities (#22694)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22694
Move quantization and quantized utility functions for testing to common_quantized.py and common_quantization.py. Addditionally, add a quantized test case base class which contains common methods for checking the results of quantization on modules. As a consequence of the move, fixed the import at the top of test_quantized.py, and test_quantization to use the new utility
Reviewed By: jerryzh168
Differential Revision: D16172012
fbshipit-source-id: 329166af5555fc829f26bf1383d682c25c01a7d9
