[caffe2][easy] Save heap allocation in ConcatOp (#63529)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63529
Output() takes an IntArrayRef, so we can just use a std::initializer_list (stack-allocated array) instead of std::vector here.
ghstack-source-id: 137085908
Test Plan: existing CI
Reviewed By: mruberry
Differential Revision: D29687400
fbshipit-source-id: 9f2a7c6679f2552c098bb1bf7befaca18e0e5d4d
