fold col offset into bias; optimize A symmetric quant (#17026)

fold col offset into bias; optimize A symmetric quant (#17026)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17026

D14013931 was for FC. This diff is similar optimizations for Conv.
A subtle difference is that in FC, once we fold col_offset into bias during pre-processing step, we can treat everything as if A_zero_offset == 0 (symmetric quantization of A).
In Conv, we can't do this because padding still needs to use the original A_zero_offset.
From requantization point of view, once col_offset folded into bias, we can treat as if we're doing symmetric A quantization.
But, for steps involving padding like im2col, im2col fused with packing, and direct conv for depth-wise/group convolution we still need to pass the original A_zero_offset.

Reviewed By: jianyuh

Differential Revision: D14020276

fbshipit-source-id: c29caefd1127bbc6aff0e9d535939bb0c1ecb66c