[JIT] Add variadic stack op (#63578)

[JIT] Add variadic stack op (#63578)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63578

Added a new op `prim::VarStack` and a pass that transforms instances of `aten::stack(list, dim)` into `prim::VarStack(list[0], ..., list[n], dim)`. Also provided a JIT interpreter implementation.

Most of the implementation/tests are the same as `prim::VarConcat`.

Test Plan: `buck test caffe2/test/cpp/jit:jit -- TestStackOpt`

Reviewed By: navahgar

Differential Revision: D30426232

fbshipit-source-id: 9829a7db6e0a5038c9b7528c43c25b0c221aa2ce