Fix ONNX RuntimeError with dynamic WorldModel (#19322)

Signed-off-by: Mohammed Yasin <32206511+Y-T-G@users.noreply.github.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/nn/tasks.py

@@ -637,8 +637,8 @@ class WorldModel(DetectionModel):
             (torch.Tensor): Model's output tensor.
         """
         txt_feats = (self.txt_feats if txt_feats is None else txt_feats).to(device=x.device, dtype=x.dtype)
-        if len(txt_feats) != len(x):
-            txt_feats = txt_feats.repeat(len(x), 1, 1)
+        if len(txt_feats) != len(x) or self.model[-1].export:
+            txt_feats = txt_feats.expand(x.shape[0], -1, -1)
         ori_txt_feats = txt_feats.clone()
         y, dt, embeddings = [], [], []  # outputs
         for m in self.model:  # except the head part