quant docs: add common errors section (#49902)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49902
Adds a common errors section, and details the two errors
we see often on the discuss forums, with recommended solutions.
Test Plan: build the docs on Mac OS, the new section renders correctly.
Reviewed By: supriyar
Differential Revision: D25718195
Pulled By: vkuzo
fbshipit-source-id: c5ef2b24831d18d57bbafdb82d26d8fbf3a90781
