[JIT] UseVariadicOp takes list_idx parameter (#63915)

[JIT] UseVariadicOp takes list_idx parameter (#63915)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63915

Previously, this function only worked for variadic op substitutions of the form `op(list, args) -> variadic_op(list_1, ..., list_n, args)`. This change allows for transformations of the form `op(args_0, list, args_1) -> variadic_op(args_0, list_1, ..., list_n, args_1)`.

Test Plan:
`buck test caffe2/test/cpp/jit:jit -- Stack Concat`

(tests exercising `list_idx != 0` will be added further up in this diff stack)

Reviewed By: navahgar

Differential Revision: D30529729

fbshipit-source-id: 568080679c3b40bdaedee56bef2e8a5ce7985d2f