[QNN EP] Fix error handling for Softmax/ReduceOps (#16700)
### Description
- Fix check for Softmax with axis attributes not equal to -1. QNN EP
only supports axis values equal to -1 (or rank - 1).
- Explicit error when Reduce* ops have an input with rank > 4 on HTP
backend (unsupported).
- Correctly filter out partitions that only contain a single
QuantizeLinear or DequantizeLinear node.
- Add tests for the above and clean up unnecessary usage of test
description labels.
### Motivation and Context
Make it easier to debug why a model may not be supported.
