Honor model_external_initializers_file_folder_path for file-path model loads

[jambayk]

Description

session.model_external_initializers_file_folder_path now takes effect for every model load path and overrides the model's own directory as the base for resolving external initializers.

• New Model::Load(file_path, graph_model_path, ...) overload reads the model bytes from file_path while storing a separate graph model path that is used only for external-data resolution.
• InferenceSession applies the option in all ONNX load paths (file path, memory buffer, ModelProto, istream, and the parsed-proto Load()): the model is read from its real location, but external initializers are resolved from the option's folder. The prior restriction that only honored the option when the model location was empty is removed.

Adds a shared-lib test that loads a model from a path whose directory does not contain the external data and resolves it via the option.

Motivation and Context

The option was originally added in #23557 to let a model loaded from an in-memory buffer point ORT at a folder holding its external initializer files, since a buffer has no directory of its own. That implementation only applied when the model location was empty, so it worked exclusively for buffer/stream loads and required the caller to clear the model directory first.

Since then the option is used for more scenarios: weightless/cache models, models that share a single weights file, and packaged models whose weights live outside the model's own folder. In all of these the caller wants to point ORT at a specific external-initializers folder regardless of how the model itself is loaded. Restricting it to buffer loads made the usage narrow and unexpected.

This change makes the behavior generic: the option is honored for file-path loads too and overrides the model directory. Overriding is safe because the option only changes the base directory used to resolve external initializer files; the model bytes are still read from the real location. Downstream consumers of the graph model path already tolerate a virtual or non-existent model path, since buffer loads (a very common case) have always produced exactly such a path, and they either use only the parent directory or guard file access with existence checks.

A follow-up PR will build on this to simplify the model package flow (fold the external_data variant field into general session-options path resolution) and to remove the legacy directory-based CreateSession path.

[Copilot]

Pull request overview

This PR updates ONNX model loading so session.model_external_initializers_file_folder_path is honored for all load paths (including file-path loads), overriding the model’s directory when resolving external initializer data while still reading model bytes from the original location.

Changes:

• Added a Model::Load(file_path, graph_model_path, ...) overload to decouple “where bytes are read from” vs “which path is used for external-data resolution”.
• Updated InferenceSession ONNX load paths to apply model_external_initializers_file_folder_path uniformly (file path, memory buffer, ModelProto, istream, parsed-proto Load()).
• Added a shared-lib test that validates external initializers can be resolved from an override folder even when the model is loaded from a path in a different directory.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
onnxruntime/test/shared_lib/test_model_loading.cc	Adds a new test covering path-based loads with external-initializer folder override.
onnxruntime/core/session/inference_session.cc	Applies the external-initializers folder override across all ONNX load entrypoints.
onnxruntime/core/graph/model.h	Declares a new Model::Load overload that accepts a separate graph model path for external-data resolution.
onnxruntime/core/graph/model.cc	Implements the new Model::Load(file_path, graph_model_path, ...) overload via LoadModelHelper.
include/onnxruntime/core/session/onnxruntime_session_options_config_keys.h	Updates documentation for session.model_external_initializers_file_folder_path to reflect broader behavior.