Fix shape inference when auto_pad is notset again (#1830)
Fix shape inference when auto_pad is notset again (#1830)

Treat auto_pad notset as no auto_pad,
since our newest Modelzoo onnx models still make auto_pad notset instead of removing it.
onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx