[SR] Migrate gather_ranges_to_dense to new FuseListUnpack (#67164)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67164
Migrated both the variadic and non-variadic versions.
This diff is part of the effort to migrate all ops used in `FuseListUnpack` to `FuseListUnpackV2`. The original version of `FuseListUnpack` is problematic for a few reasons:
* You have to complicate the op implementation with an `is_fused` check, resulting in messier code. It is easier to reason about two ops, fused (out variant) and unfused (native).
* The original version of `FuseListUnpack` is buggy. It assumes that the `ListUnpack` node occurs immediately after the fusion candidate, which is not necessarily true.
This diff finishes the migration, so the original version of `FuseListUnpack` is removed
Test Plan:
Unit tests: `buck test caffe2/benchmarks/static_runtime/...`
**Accuracy Test**
Done at the top of this diff stack.
Reviewed By: hlu1
Differential Revision: D31887386
fbshipit-source-id: 9d44c813667a75bce13dce62bf98e6109edea6ba
