Fix exporting ONNX model with fixed batch sizes (#508) (#509)

yolort/relay/trt_graphsurgeon.py

@@ -75,7 +75,10 @@ def __init__(
             logger.info(f"Loaded saved model from {model_path}")
 
             if input_sample is not None:
+                self.batch_size = input_sample.shape[0]
                 input_sample = input_sample.to(device=device)
+            else:
+                self.batch_size = 1
             model_path = model_path.with_suffix(".onnx")
             model.to_onnx(model_path, input_sample=input_sample, enable_dynamic=enable_dynamic)
             logger.info("PyTorch2ONNX graph created successfully")
@@ -85,7 +88,6 @@ def __init__(
 
         # Fold constants via ONNX-GS that PyTorch2ONNX may have missed
         self.graph.fold_constants()
-        self.batch_size = 1
         self.precision = precision
         self.simplify = simplify