Removes SymbolicVariable (#25077)
Summary:
This PR excises the last of SymbolicVariable. There should be no change in functionality. One new test for addmm fusion was added. A case where the peephole optimizer might convert a scalar argument remains untested.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25077
Test Plan: Refactors existing code so mostly covered by current tests. One test for addmm fusion was added.
Differential Revision: D17145334
Pulled By: mruberry
fbshipit-source-id: 6b68faf764f9ee8398b55c43110228ed9faf81eb
