Update weight sharing tool to support plugin EPs

[adrianlizarraga]

Description

• Updates the ep_weight_sharing_ctx_gen tool to support specifying a plugin EP configuration (via JSON).
• Mark the ep_weight_sharing_ctx_gen tool as deprecated and add notification to README that recommends the use the public Python ORT APIs instead.
  • Note we no longer publish a binary for this tool as of ORT 1.22.2.
• Added an example Python script in the README.
• Added a Python unit test that tests compiling models with weight sharing using an example plugin EP.

Tool usage

Create a JSON file that contains information about the plugin EP to load/use (e.g., example_plugin_ep_config.json):

{
    "ep_library_registration_name": "example_plugin_ep",
    "ep_library_path": "example_plugin_ep.dll",
    "selected_ep_name": "example_plugin_ep",
    "default_ep_options": { "option_key": "option_value" }
}

Call the ep_weight_sharing_ctx_gen tool with the -p command-line option to specify the location of the above configuration file:

$ ep_weight_sharing_ctx_gen.exe -p example_plugin_ep_config.json model_1.onnx,model_2.onnx

Motivation and Context

Close the functionality gap between traditional provider-bridge EPs and plugin EPs. This PR allows using plugin EPs with the tool that compiles models with weight sharing.

[adrianlizarraga]

I'm thinking we remove this tool after ORT 1.24. At that point we can consider replacing it with a Python version if we deem it necessary (although a simple example script in the docs is probably enough).

[edgchen1]

nit: optional<unique_ptr> feels unnecessary. can we just use an empty unique_ptr to represent the empty state?

[edgchen1]

there should be some non-Windows support for device discovery now