[QNN EP] Fix error handling for Softmax/ReduceOps

[adrianlizarraga]

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.

[HectorSVC]