[WebNN EP] Optimize model partitioning (#23332)
### Description
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The old `GetCapability` function of WebNN EP is just a very simple
search for groups of nodes that can be handled. This doesn't work well
in the following example graph, where A and D could be handled by the
EP, but B is between them in the topological order, as you get two
single node capabilities. However, it may also be advantageous if C and
E could be handled by the EP, since they would be combined with D even
though they are not connected.
```
A B C
| / |
D E
| |
```
Therefore, we improve partitioning results by reusing
`utils::CreateSupportedPartitions`, which walks the edges for each node
that the EP can handle as they are iterated in topological order. This
would guarantee that all connected nodes that can be handled are grouped
together. Correspondingly, we modify the `webnn::GetSupportedNodes`
function to return the supported nodes instead of the group of supported
partitions.
### 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. -->
Co-authored-by: Dwayne Robinson <fdwr@hotmail.com>
