Make nnapi flatten converter accept flex inputs (#61024)
Summary:
As title
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61024
Test Plan: pytest test/test_nnapi.py::TestNNAPI::test_flatten
Reviewed By: anshuljain1
Differential Revision: D29480748
fbshipit-source-id: c334b09600a64d3e552cec843d6da3de28e7d27c
