[SR] Fast path for VarStack on scalars (#70210)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70210
Add a fast-path for `VarStack` nodes for when the inputs are scalars.
Test Plan: `buck test caffe2/benchmarks/static_runtime:static_runtime_cpptest -- VarStack`
Reviewed By: hlu1
Differential Revision: D33177498
fbshipit-source-id: 922ab76a6808fbfdb8eb6091163a380344e38de6
