Model package: fold external_data into session options and remove legacy directory load path

[jambayk]

Description

Two changes to the model package flow, enabled by the file-path external initializers support:

1. Fold external_data into session_options with path resolution.

• Path-valued session options (an allowlist: session.model_external_initializers_file_folder_path and ep.context_file_path) are resolved against the package (sha256:<hex>, relative, or absolute) at variant-parse time, using the same rules as model_file.
• The dedicated external_data variant field and its special session injection are removed; the model is always loaded from the selected variant path.
• On the advanced path (caller supplies their own OrtSessionOptions), path-valued options are carried over from the variant for keys the caller did not set, so a model that needs its external-initializers folder still loads.

2. Remove the legacy directory-based CreateSession(package_dir) path.

• Removes the is_directory(package_root) branch in CreateSessionAndLoadModelImpl. Model packages are loaded through the experimental OrtModelPackageApi (CreateModelPackageContext -> SelectComponent -> CreateSession).
• The three end-to-end tests that exercised the directory path are migrated to the experimental API via a CreateSessionFromModelPackage test helper, preserving coverage for factory-based selection, PREFER_CPU policy selection, and compiled-model compatibility scoring.

Motivation and Context

The file-path external initializers folder support (#29459) lets the model package flow drop its workaround of memory-mapping the model and forcing a buffer load: it sets the folder option and loads from the selected variant path directly. Building on that, the dedicated external_data field folds into the general session-options mechanism. A variant declares session.model_external_initializers_file_folder_path (or other path-valued options) in its session_options, and ORT resolves those values against the package at parse time. This removes special-case code and lets other path-valued options such as the EPContext file path be resolved the same way. An allowlist is used rather than value-syntax sniffing because session-option values are arbitrary strings; only known path-valued keys are resolved, so ordinary values pass through untouched.

The legacy directory-based CreateSession(package_dir) path was meant to be removed when the experimental OrtModelPackageApi landed. It only did variant/EP selection and never merged the variant's session_options/provider_options, so a package loaded via Ort::Session(env, dir, so) silently ignored its manifest session options, inconsistent with the experimental API. Removing it leaves a single, consistent way to load a model package.

[Copilot]

Pull request overview

This PR simplifies the model package loading flow by (1) eliminating the dedicated external_data manifest field in favor of resolving path-valued session_options entries at variant-parse time, and (2) removing the legacy OrtCreateSession(package_dir) directory-based model package load path in favor of the experimental OrtModelPackageApi pipeline.

Changes:

• Resolve allowlisted path-valued session options (e.g., session.model_external_initializers_file_folder_path, ep.context_file_path) against the package during variant parsing, and carry over those options on the “advanced” session-options path when unset by the caller.
• Remove the legacy directory-based model package load path from CreateSessionAndLoadModelImpl.
• Migrate existing end-to-end tests to create sessions via the experimental OrtModelPackageApi, and add a new test covering external-initializers folder resolution via variant session options.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
onnxruntime/test/autoep/test_model_package.cc	Adds a helper to create sessions via OrtModelPackageApi, migrates directory-path tests, and adds a new test for resolved external initializers folder session option.
onnxruntime/core/session/utils.cc	Removes directory-based model package loading from the standard session creation path.
onnxruntime/core/session/model_package/README.md	Updates manifest docs to move external_data behavior into session_options and documents path-valued option resolution and carryover semantics.
onnxruntime/core/session/model_package/model_package_context.h	Removes legacy external-data fields and introduces IsModelPackagePathSessionOption.
onnxruntime/core/session/model_package/model_package_context.cc	Implements the allowlist and resolves path-valued session options during variant parsing.
onnxruntime/core/session/model_package_api.cc	On advanced path, carries over only variant path-valued session options that the caller didn’t set.