[quant] Fix qmin/qmax when using customized qrange (#74717)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74717
currently the weight map to 0 and max_float to 65535 due to incorrect qmin/qmax in qin16 customized qrange
the expectation from the set observers is the integer representation is supposed to be a signed int16 i.e -32768 to 32767.
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D35129924
fbshipit-source-id: 924902dd7e64c1218971422ba2451c2a484fd2f4
(cherry picked from commit 95659cdeeec7b3a01a64355244847e211c6dd2a6)
