add test for seed variation in sampling

Add test for seed variation in sampling with temperature > 0.

tests/test_engine.py

@@ -233,3 +233,28 @@ def test_num_samples_count():
     for num_samples in [1, 4, 16, 64]:
         results, _ = engine.generate_batch(prompt, num_samples=num_samples, max_tokens=3)
         assert len(results) == num_samples, f"Expected {num_samples} sequences from {num_samples} samples, got {len(results)}"
+
+
+def test_seed_variation_in_sampling():
+    """With temperature > 0, different seeds should introduce sampling variation."""
+    model = MockModel()
+    engine = Engine(model, ByteTokenizer())
+    prompt = [261, 72, 101, 108, 108, 111]  # <bos> + "Hello"
+
+    outputs = set()
+
+    for seed in [1, 42, 123, 999, 1000, 1001, 1002, 1003, 1004, 1005]:
+        results, _ = engine.generate_batch(
+            prompt,
+            temperature=1.0,
+            max_tokens=5,
+            seed=seed,
+        )
+        outputs.add(tuple(results[0]))
+
+    # Sanity check: sampling actually introduces variation
+    assert len(outputs) > 1, (
+        f"All seeds produced the same output: {outputs}"
+        f"with temperature > 0 and different seeds, this is statistically impossible."
+        f"implies an issue within engine."
+    )