Add preprocessing that fuses decomposed linear into linear. (#37937)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37937
Sometime traces models dont preseve aten::linear ops and they are decomposed
into addmm or mul + add. Adding thie preprocessing step helps us catch more
lowerable linear nodes.
Please see the test for example.
Test Plan: python test/test_xnnpack_integration.py
Reviewed By: xcheng16
Differential Revision: D21428069
fbshipit-source-id: 6c4ea3335eaf5722852c639fb4ee593746bb408f
