Pass folder path to store initializers as files

[wschin]

One solution for #493. It requires small PyTorch exporter change to passing the newly added parameter from user code -> pytorch-to-onnx exporter -> onnxscript.

This should replace #930 as a final solution of #493.

[BowenBao]

I'm thinking about making the api to return model_proto and initializers separately. It is similar to what you did with symbolic_trace. The model_proto and initializers are only written to disk on the save/serialize call.

# model_proto frame and initializers are still in memory, inside export_output.
export_output = torch.onnx.dynamo_export(model, ...)
# Now we have the path, serialize model to there.
export_output.save("path_to_model.onnx")

[justinchuby]

Added comments in #493.