onnxruntime

Improve handling of symbolic dimensions in the onnxruntime_test.py script. (#3959)

If a symbolic dimension is found allow the user to provide a value, or default to 1.

`python .\onnxruntime_test.py --symbolic_dims batch=1,seqlen=4 onnxruntime\test\testdata\transform\fusion\fast_gelu_use_graph_input.onnx`