onnxruntime

Add support to save onnx graph with external initializers file. (#6911)

Add functionality to the Graph class to be dumped to protobuf using an external binary file for the float initializers.

This change is meant to avoid hitting the 2GB protobuf limit when dumping large graphs.
This limit was particularly easy to exceed when dumping graphs after auto-diff.
The use of the external file is limited to initializers larger than a user-specified threshold.
This gives the possibility to users to include in the onnx file shape constants used by Reshape and Transpose used by Shape Inference.