[Static Runtime] Implement prim::VarStack out variant (#63579)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63579
Provide a static runtime out variant implementation for the new op introduced in D30426232 (https://github.com/pytorch/pytorch/commit/1385f9fb12e6607c98d2d9d5edaaaab2bc07386f).
Test Plan: `buck test //caffe2/benchmarks/static_runtime:static_runtime_cpptest -- IndividualOps_VarStack`
Reviewed By: navahgar
Differential Revision: D30410525
fbshipit-source-id: bc59a3d8ad23e3d94561ec2dca9cc20687dbadf8
