Fix some issues with NNAPI Softmax (#16095)
### Description
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Update NNAPI Softmax to coerce to 2D when opset is < 13. This prevents
the layout change to NHWC from breaking the implementation, as well as
making it work correctly when the ONNX node's axis != 1.
Add check for opset 13+ that axis is inner-most dimension as we don't
currently handle any other value correctly.
Update tests to add model to check NHWC layout, as well as 4D tests. We
didn't notice the issues with the NNAPI EP as it was only processing
input shapes that were 2D or 4D (which was overly restrictive as well).
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
#15949
