Remove prim::Constant op (#32804)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32804
Constants are interpreter primitives so the op was not actually used.
This cleans up some of the logic around it.
This also fixes constant prop such that failures to look up an op
do not silently stop constant propagation. Instead, only errors
inside the op implementation itself will do this.
Test Plan: Imported from OSS
Differential Revision: D19673156
Pulled By: zdevito
fbshipit-source-id: 7beee59a6a67a6c2f8261d86bd505280fefa999e
