[#10696][fix] AutoDeploy prevent torch.export from specializing batch dimension when max_batch_size=1 (#10697)

Signed-off-by: Eran Geva <19514940+MrGeva@users.noreply.github.com>

tensorrt_llm/_torch/auto_deploy/custom_ops/attention_interface.py

@@ -725,7 +725,9 @@ class SequenceInfo:
         """Set an example sequence useful for testing and export purposes without cache history."""
         # use a best guess default for input_ids if not provided
         if input_ids is None:
-            bs, seq_len = min(2, self.max_batch_size), min(4, self.max_seq_len)
+            # Use batch_size >= 2 for export to prevent torch.export from specializing
+            # the batch dimension when max_batch_size=1 (dimension value 1 triggers static optimization)
+            bs, seq_len = max(2, min(2, self.max_batch_size)), min(4, self.max_seq_len)
             input_ids = torch.ones(bs, seq_len, dtype=torch.int).tolist()
 
         # figure out page assignments

tests/unittest/_torch/auto_deploy/unit/singlegpu/shim/test_llm_config.py

@@ -230,3 +230,49 @@ def test_attention_backend_page_size_logic(backend, expected_attn_page_size):
         transforms={"insert_cached_attention": {"stage": "cache_init", "backend": backend}},
     )
     assert args.attn_page_size == expected_attn_page_size
+
+
+class TestSequenceInfoExampleBatchSize:
+    """Test that SequenceInfo generates proper example batch sizes for export."""
+
+    def test_example_batch_size_at_least_2_when_max_batch_size_1(self):
+        """Test that example batch size is at least 2 even when max_batch_size=1.
+
+        This is critical because torch.export specializes dimensions when the
+        example input has a dimension value of 1, breaking dynamic batching.
+        """
+        from tensorrt_llm._torch.auto_deploy.custom_ops.attention_interface import SequenceInfo
+
+        seq_info = SequenceInfo(
+            max_batch_size=1,
+            max_seq_len=128,
+            max_num_tokens=128,
+            page_size=64,
+        )
+
+        # Set example sequence (this is what's used during export)
+        seq_info.set_example_sequence()
+
+        # The example batch size should be at least 2 to prevent torch.export
+        # from specializing the batch dimension
+        assert len(seq_info.named_args["input_ids"]) >= 2, (
+            f"Example batch size should be >= 2 for export, got {len(seq_info.named_args['input_ids'])}"
+        )
+
+    def test_example_batch_size_normal_max_batch_size(self):
+        """Test example batch size with normal max_batch_size."""
+        from tensorrt_llm._torch.auto_deploy.custom_ops.attention_interface import SequenceInfo
+
+        seq_info = SequenceInfo(
+            max_batch_size=32,
+            max_seq_len=128,
+            max_num_tokens=128,
+            page_size=64,
+        )
+
+        seq_info.set_example_sequence()
+
+        # With larger max_batch_size, example should still be 2
+        assert len(seq_info.named_args["input_ids"]) == 2, (
+            f"Expected example batch size of 2, got {len(seq_info.named_args['input_ids'])}"
+        )