Fix shape inference failure with file-backed external data

[yoonseok-kim]

Description

This change allows ONNX shape inference to consume file-backed external initializers when their values are required.

• Materialize the requested external initializer into a temporary TensorProto.
• Keep the original graph initializer external so runtime loading can continue to use the file-backed data.
• Reuse the existing temp_tensor_protos_ lifetime, so the temporary data is released after node shape inference.
• Add a graph test that stores ReduceSum.axes in external data, loads the model, verifies the inferred output shape, and confirms that the graph initializer remains external.

Motivation and Context

ONNX models can store initializer payloads in separate external-data files. This includes offline-optimized artifacts saved by ONNX Runtime as well as models created or repackaged by other tooling. Such a model can place values required by shape inference, such as Reshape.shape, in an external-data file.

When the model is loaded again, InferenceContextImpl::getInputData() returns a TensorProto whose payload is still external. ONNX shape inference does not load file-backed external data, so loading fails with a ShapeInferenceError even though the artifact was successfully produced by ONNX Runtime.

This is the failure reported in #21325 for an offline-optimized model, but the underlying issue is not specific to offline optimization. A similar failure was previously reported in #11137.

ONNX Runtime already materializes in-memory external initializers into temporary TensorProto values for shape inference, as implemented in #26263. This change extends the same approach to file-backed external initializers. Only initializer values requested by shape inference are read; the original graph initializer and the runtime file-backed loading path are unchanged.

Fixes #21325

Testing

• Added GraphTest.ShapeInferenceWithFileBackedExternalData, which verifies that:
  • the inferred ReduceSum output shape is correct; and
  • the graph initializer remains file-backed after model loading.

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