updates

llms/mlx_lm/tuner/grpo_trainer.py

@@ -164,6 +164,42 @@ def r1_count_xml(prompts: list, completions: list, answer: list, **kwargs) -> li
     return scores
 
 
+def get_per_token_logps(model, inputs, lengths):
+    # Get logits from model
+    logits = model(inputs).astype(mx.float32)  # [batch_size, seq_len, vocab_size]
+    # Remove last position as it corresponds to the next token prediction
+    logits = logits[:, :-1, :]  # [batch_size, seq_len-1, vocab_size]
+    targets = inputs[:, 1:]  # Shift inputs to get targets
+    
+    # Process sequences individually to save memory
+    per_token_logps = []
+    for i in range(logits.shape[0]):
+        # Get sequence length for this example
+        seq_len = int(lengths[i]) - 1  # -1 because we removed last position
+        
+        # Get logits and targets for this sequence
+        seq_logits = logits[i, :seq_len]  # [seq_len, vocab_size]
+        seq_targets = targets[i, :seq_len]  # [seq_len]
+        
+        # Compute log probabilities
+        log_probs = nn.log_softmax(seq_logits, axis=-1)  # [seq_len, vocab_size]
+        
+        # Gather log probs for actual tokens
+        token_log_probs = mx.take_along_axis(
+            log_probs,
+            seq_targets.reshape(seq_len, 1),
+            axis=-1
+        ).squeeze(-1)  # [seq_len]
+        
+        per_token_logps.append(token_log_probs)
+        
+        # Clean up intermediates
+        del seq_logits, seq_targets, log_probs, token_log_probs
+        mx.metal.clear_cache()
+    
+    return per_token_logps
+
+
 def grpo_loss(
     model,
     tokenizer,
@@ -248,24 +284,30 @@ def grpo_loss(
     targets = inputs[:, 1:]
     
     # Current policy probabilities
-    token_log_probs = mx.take_along_axis(
+    token_log_probs = get_per_token_logps(model, inputs, lengths)
-        log_probs,
-        targets.reshape(*targets.shape, 1),
-        axis=-1
-    ).squeeze(-1)
     
     # Reference policy probabilities
     if ref_model is not None:
-        ref_logits = ref_model(inputs).astype(mx.float32)
+        ref_token_log_probs = get_per_token_logps(ref_model, inputs, lengths)
     else:
-        ref_logits = mx.array(logits)
+        ref_token_log_probs = token_log_probs
         
-    ref_log_probs = nn.log_softmax(ref_logits[:, :-1, :], axis=-1)
+    max_len = max(x.shape[0] for x in token_log_probs)
-    ref_token_log_probs = mx.take_along_axis(
+    padded_log_probs = []
-        ref_log_probs,
+    padded_ref_log_probs = []
-        targets.reshape(*targets.shape, 1),
+
-        axis=-1
+    for i in range(len(token_log_probs)):
-    ).squeeze(-1)
+        seq_len = token_log_probs[i].shape[0]
+        padding = mx.zeros((max_len - seq_len,), dtype=mx.float32)
+        
+        padded_log_probs.append(mx.concatenate([token_log_probs[i], padding]))
+        padded_ref_log_probs.append(mx.concatenate([ref_token_log_probs[i], padding]))
+        
+        del padding
+        mx.metal.clear_cache()
+    
+    token_log_probs = mx.stack(padded_log_probs)
+    ref_token_log_probs = mx.stack(padded_ref_log_probs)
     
     # Calculate rewards and advantages
     rewards = mx.zeros((len(all_completions),))