[fx] remove delegate, replace with tracer (#44566)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44566
The Delegate objects were confusing. They were suppose to be a way to
configure how tracing works, but in some cases they appeared necessary
for consturcting graphs, which was not true. This makes the organization
clearer by removing Delgate and moving its functionality into a Tracer class,
similar to how pickle has a Pickler class.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23683177
Pulled By: zdevito
fbshipit-source-id: 7605a34e65dfac9a487c0bada39a23ca1327ab00
