Fix ONNX opset inconsistency with parseargs and run args (ultralytics#5937)

Author: d57montes

export.py

@@ -325,7 +325,7 @@ def run(data=ROOT / 'data/coco128.yaml',  # 'dataset.yaml path'
         int8=False,  # CoreML/TF INT8 quantization
         dynamic=False,  # ONNX/TF: dynamic axes
         simplify=False,  # ONNX: simplify model
-        opset=12,  # ONNX: opset version
+        opset=14,  # ONNX: opset version
         verbose=False,  # TensorRT: verbose log
         workspace=4,  # TensorRT: workspace size (GB)
         topk_per_class=100,  # TF.js NMS: topk per class to keep