Make tracer be able to trace different forward functions (#77109)
Summary: The root module may have different forward functions. The current implementation assumes only the func `forward` can be traced. In this diff, we add an argument of forward func name to enable users trace different forward functions
Test Plan: N1903198
Differential Revision: D36157032
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77109
Approved by: https://github.com/jamesr66a
