[JIT] Move UseVariadicCat internals (#63577)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63577
Since other variadic ops will have an almost identical implementation, we can generalize the `UseVariadicCat` implementation and put it in a common folder.
Also moved some test utilities that other variadic op tests will likely need.
Test Plan: `buck test caffe2/test/cpp/jit:jit -- ConcatOptTest`
Reviewed By: navahgar
Differential Revision: D30409937
fbshipit-source-id: 925c11c27b58ce98cb8368d2a205e26ba66d3db9
