[QNN EP] Support prelu fp16 (#20428)
### Description
Originally, Prelu in QNN will fail when the input is fp16 and alpha is fp32.
QNN requires alpha is fp16 when input is fp16.
This can be resolved by casting alpha to fp16 and pass it to QNN.
### Motivation and Context
Makes QNN Prelu support fp16 case.
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Co-authored-by: Hector Li <hecli@microsoft.com>
