Move to ProxyValue instead of FakeTensor weights. (#14697)

Summary:

With pt2, weights show up as ProxyValue instead of FakeTensor. This diff gets rid of old code that checks if weights are FakeTensor/ProxyValue and has logic to handle the two separately.

Reviewed By: zonglinpeng, ethansfng, DrJessop

Differential Revision: D82605179

Author: hsharma35