[ET-VK] Fix op benchmarks code generation (#6528)

## Context

As title. There was a small error in the operator benchmark code generation which impacted operators which return a collection of tensors (e.g. layer norm). The issue was that the generated code would store the output `ValueRefs` in variables that shadowed class variables of the benchmark.

Differential Revision: [D65071024](https://our.internmc.facebook.com/intern/diff/D65071024/)

ghstack-source-id: 250448840
Pull Request resolved: https://github.com/pytorch/executorch/pull/6527

Co-authored-by: Stephen Jia <ssjia@meta.com>