bugfix: string_view of invalid memory (#23417)
### Description
the `std::unordered_map` uses a `std::string_view` as key, while the
string view may refer to invalid memory. Function `IdentityBuilder`
returns a `std::string` which goes out of scope quickly.
```c++
unordered_map<string_view, std::vector<NodeIndex>> identical_children_map;
for (auto i = node->OutputEdgesBegin(); i != node->OutputEdgesEnd(); ++i) {
if (i->GetNode().OpType() == op) {
identical_children_map[IdentityBuilder(graph, i->GetNode())].push_back(i->GetNode().Index());
}
}
```
This code will cause a waring as error in EMSDK v4.0.1:
```
C:/code/o2/onnxruntime/core/optimizer/identical_children_consolidation.cc:51:30: error: object whose reference is captured by 'identical_children_map' will be destroyed at the end of the full-expression [-Werror,-Wdangling-capture]
51 | identical_children_map[IdentityBuilder(graph, i->GetNode())].push_back(i->GetNode().Index());
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```
