Set `dynamic` to True only if imgsz is dynamic for ONNX (#17872)

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Co-authored-by: Ultralytics Assistant <135830346+UltralyticsAssistant@users.noreply.github.com>

ultralytics/engine/predictor.py

@@ -155,7 +155,7 @@ class BasePredictor:
         same_shapes = len({x.shape for x in im}) == 1
         letterbox = LetterBox(
             self.imgsz,
-            auto=same_shapes and (self.model.pt or getattr(self.model, "dynamic", False)),
+            auto=same_shapes and (self.model.pt or (getattr(self.model, "dynamic", False) and not self.model.imx)),
             stride=self.model.stride,
         )
         return [letterbox(image=x) for x in im]