[EP ABI] OrtGraphToProto utils fixes (#25531)
### Description
Fixes for the OrtGraphToProto utilities that EPs can copy and modify:
- When serializing `OrtGraph` to ONNX protobuf, do not set an
`onnx::TensorShapeProto` for `onnx::ValueInfo` if the shape has no
dimension entries. Otherwise, the shape incorrectly looks like a scalar.
- Add `ORT_OP_ATTR_GRAPH` to the enum values returned by the
`OpAttr_GetType` C API function. This allows the OrtGraphToProto
utilities to skip processing subgraph attributes, which can be retrieved
via a different API, but return an error on any unsupported attribute
type.
### Motivation and Context
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