Modify ONNX constant folding test point in test_utility_funs.py for clarity (#28861)
Summary:
This is a minor update to the test point `TestUtilityFuns.test_constant_fold_concat` in `test/onnx/test_utility_fun.py` for clarity. Unlike before, the test model forward() method now uses the input `x`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28861
Differential Revision: D18306881
Pulled By: houseroad
fbshipit-source-id: dda8b4123e7646c2e416ce914a4698f9b96e2a6c
