[WebNN EP] Fix bug in Softmax (#17665)
### Description
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For now, WebNN Softmax only support 2D (or implicitly coerce to 2D)
inputs and the last axis.
### 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. -->
Fallback some cases to pass the CI.
