batched min p and fix spec gen sampling (#1222)

llms/mlx_lm/sample_utils.py

@@ -147,11 +147,11 @@ def min_p_sampling(
     logprobs = logprobs * (1 / temperature)
 
     # Indices sorted in decreasing order
-    sorted_indices = mx.argsort(-logprobs).squeeze(0)
-    sorted_logprobs = logprobs[..., sorted_indices]
+    sorted_indices = mx.argsort(-logprobs, axis=-1)
+    sorted_logprobs = mx.take_along_axis(logprobs, sorted_indices, axis=-1)
 
     # Top probability
-    top_logprobs = logprobs[..., sorted_indices[0]]
+    top_logprobs = sorted_logprobs[:, 0:1]
 
     # Calculate the min_p threshold
     scaled_min_p = top_logprobs + math.log(min_p)
@@ -163,9 +163,9 @@ def min_p_sampling(
     # Create pool of tokens with probability less than scaled min_p
     selected_logprobs = mx.where(tokens_to_remove, -float("inf"), sorted_logprobs)
 
-    # Return sampled token
-    sorted_token = mx.random.categorical(selected_logprobs)
-    return sorted_indices[sorted_token]
+    # Return sampled tokens
+    sorted_tokens = mx.random.categorical(selected_logprobs, axis=-1)[:, None]
+    return mx.take_along_axis(sorted_indices, sorted_tokens, axis=-1).squeeze(1)
 
 
 @partial(mx.compile, inputs=mx.random.state, outputs=mx.random.state)
@@ -185,7 +185,7 @@ def top_p_sampling(logits: mx.array, top_p: float, temperature: float) -> mx.arr
 
     # sort probs in ascending order
     sorted_indices = mx.argsort(probs, axis=-1)
-    sorted_probs = probs[..., sorted_indices.squeeze(0)]
+    sorted_probs = mx.take_along_axis(probs, sorted_indices, axis=-1)
 
     cumulative_probs = mx.cumsum(sorted_probs, axis=-1)
 
@@ -196,10 +196,8 @@ def top_p_sampling(logits: mx.array, top_p: float, temperature: float) -> mx.arr
         0,
     )
 
-    sorted_token = mx.random.categorical(mx.log(top_probs))
-    token = sorted_indices.squeeze(0)[sorted_token]
-
-    return token
+    sorted_tokens = mx.random.categorical(mx.log(top_probs), axis=-1)[:, None]
+    return mx.take_along_axis(sorted_indices, sorted_tokens, axis=-1).squeeze(1)
 
 
 @partial(mx.compile, inputs=mx.random.state, outputs=mx.random.state)

llms/mlx_lm/utils.py

@@ -398,8 +398,9 @@ def speculative_generate_step(
             quantize_cache_fn(cache)
 
             logprobs = logits - mx.logsumexp(logits, keepdims=True)
-            y = sampler(logprobs).squeeze(0)
-            return y, logprobs.squeeze(0)
+            logprobs = logprobs.squeeze(0)
+            y = sampler(logprobs)
+            return y, logprobs
 
     def _prefill(model, cache, y):
         while y.size > prefill_step_size:

llms/tests/test_sample_utils.py

@@ -28,6 +28,12 @@ class TestSampleUtils(unittest.TestCase):
         token = top_p_sampling(logits, 0.95, temperature).item()
         self.assertTrue(token in (1, 2, 3))
 
+        # Batch mode works
+        probs = mx.array([[0.9, 0.0, 0.0, 0.1], [0.0, 0.8, 0.0, 0.1]])
+        logits = mx.log(probs)
+        tokens = top_p_sampling(logits, 0.5, temperature)
+        self.assertEqual(tokens.tolist(), [0, 1])
+
     def test_min_p_sampling(self):
         probs = mx.array([0.9, 0.0, 0.0, 0.1])[None]
         logits = mx.log(probs)
@@ -42,6 +48,12 @@ class TestSampleUtils(unittest.TestCase):
             token = min_p_sampling(logits, 0.05)
             self.assertTrue(token in (0, 3))
 
+        # Batch mode works
+        probs = mx.array([[0.9, 0.0, 0.0, 0.1], [0.0, 0.8, 0.0, 0.1]])
+        logits = mx.log(probs)
+        tokens = min_p_sampling(logits, 0.7)
+        self.assertEqual(tokens.tolist(), [0, 1])
+
     def test_top_k_sampling(self):
         probs = mx.array([0.9, 0.0, 0.0, 0.1])[None]
         logits = mx.log(probs)