Add aten::softmax NNAPI converter (#58539)
Summary:
Add support for aten::softmax op in the NNAPI model converter with
flexible size
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58539
Test Plan: pytest test/test_nnapi.py::TestNNAPI::test_softmax
Reviewed By: anshuljain1
Differential Revision: D28531946
fbshipit-source-id: 8633f3e3f7f52795f9866ff16ad0867ea36a19e8
