Fix batch size zero for QNNPACK linear_dynamic (#40588)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40588
Two bugs were preventing this from working. One was a divide by zero
when multithreading was enabled, fixed similarly to the fix for static
quantized linear in the previous commit. The other was computation of
min and max to determine qparams. FBGEMM uses [0,0] for [min,max] of
empty input, do the same.
Test Plan: Added a unit test.
Differential Revision: D22264415
Pulled By: dreiss
fbshipit-source-id: 6ca9cf48107dd998ef4834e5540279a8826bc754
