Create script in runner to convert opset versions

[MaxBanister]

Create a script convert_to_opset9.py that takes in an ONNX model of opset version 7 as the first argument on the command line and converts it to one of version 9. The new file has the same name as the old one with a "_op9" before the file extension.

[pranav-prakash]

LGTM, merged. Thanks!
P.S. although it's probably overkill for this, you might consider using pathlib to get the .stem of the filename (instead of [:-5]) and the wonderful argparse for parsing cmd args,