[WebNN] Handle in-memory external data (#25079)
### Description
Some initializers are stored as in-memory external data, WebNN EP should
support these initializers.
### Motivation and Context
This PR:
- Added `HasExternalDataInMemory` check for external data to avoid
unexpected error.
- Wrapped the `UnpackInitializerData` to make it compatible with
external data.
Fixed #25078
