Make dim, numel, element_size into prim ops (#36551)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36551
Before, those ops were special cased in the jit codegen but that blocks our unboxing refactoring.
Instead, make those regular prim ops.
ghstack-source-id: 102081858
Test Plan: waitforsandcastle
Differential Revision: D21009196
fbshipit-source-id: b90320fce589fc0553f17582b66a5a05d0fd32d1
