Reject repetition_penalty=0 in SamplingParams.verify() (#24874)

Co-authored-by: RulinJuice <265952454+RulinJuice@users.noreply.github.com>

test/registered/unit/sampling/test_sampling_params.py

@@ -178,22 +178,36 @@ class TestSamplingParamsVerify(CustomTestCase):
 
     # --- repetition_penalty ---
     def test_repetition_penalty_negative_raises(self):
-        """Test that verify() rejects negative repetition_penalty (valid range is [0, 2])."""
+        """Test that verify() rejects negative repetition_penalty (valid range is (0, 2])."""
         sp = self._make(repetition_penalty=-0.1)
         with self.assertRaises(ValueError):
             sp.verify(self.VOCAB_SIZE)
 
+    def test_repetition_penalty_zero_raises(self):
+        """Test that verify() rejects repetition_penalty=0.
+
+        A value of 0 makes the sampling kernel divide logits by 0, producing
+        inf/NaN in the probability tensor and crashing every TP rank with a
+        device-side assert.
+        """
+        sp = self._make(repetition_penalty=0.0)
+        with self.assertRaises(ValueError):
+            sp.verify(self.VOCAB_SIZE)
+
     def test_repetition_penalty_above_two_raises(self):
         """Test that verify() rejects repetition_penalty > 2.0."""
         sp = self._make(repetition_penalty=2.1)
         with self.assertRaises(ValueError):
             sp.verify(self.VOCAB_SIZE)
 
-    def test_repetition_penalty_boundaries_valid(self):
-        """Test that boundary values 0.0 and 2.0 are both accepted."""
-        self._make(repetition_penalty=0.0).verify(self.VOCAB_SIZE)
+    def test_repetition_penalty_boundary_two_valid(self):
+        """Test that the upper boundary value 2.0 is accepted."""
         self._make(repetition_penalty=2.0).verify(self.VOCAB_SIZE)
 
+    def test_repetition_penalty_small_positive_valid(self):
+        """Test that a small positive repetition_penalty (e.g. 1e-3) is accepted."""
+        self._make(repetition_penalty=1e-3).verify(self.VOCAB_SIZE)
+
     # --- min_new_tokens / max_new_tokens ---
     def test_negative_min_new_tokens_raises(self):
         """Test that verify() rejects negative min_new_tokens."""