Use `std::string_view` directly as key to `absl::flat_hash_map::find` (#26682)
### Description
Use `std::string_view` directly as key in `find` method of
`flat_hash_map`. This part of the absl documentation may provide further
insights:
https://abseil.io/docs/cpp/guides/container#heterogeneous-lookup
### Motivation and Context
We noticed this when profiling the session creation of large models (in
terms of the number of nodes).
Signed-off-by: Christian Bourjau <christian.bourjau@quantco.com>