fix: Fix evaluation loss in KD trainer (#3271)

* fix: Fix evaluation loss in KD trainer

* Fix v2 strategy super() call

* fix: Add safety check for total_tokens in log method

* fix: simplified num items and outputs return handling

* fix: add missing model forward pass in compute_loss

* refactor: Use Template Method pattern for chat template strategies

* refactor: use pop(None) and remove v2 override

* chore: lint

---------

Co-authored-by: NanoCode012 <nano@axolotl.ai>
Co-authored-by: Wing Lian <wing@axolotl.ai>

tests/integrations/test_kd_chat_template.py

@@ -0,0 +1,81 @@
+"""
+Test for KD chat template strategies
+"""
+
+from unittest.mock import Mock
+
+import pytest
+
+from axolotl.integrations.kd.chat_template import ChatTemplateStrategyWithKDv2
+
+
+class TestChatTemplateStrategyWithKDv2:
+    """Test v2 strategy correctly handles target_token_ids"""
+
+    @pytest.fixture
+    def v2_strategy(self):
+        """Create v2 strategy instance with mocked dependencies"""
+        # Mock prompter
+        mock_prompter = Mock()
+        mock_prompter.roles = {"user": "user", "assistant": "assistant"}
+        mock_prompter.chat_template_msg_variables = ["role", "content"]
+        mock_prompter.chat_template = "{{ messages }}"
+
+        # Mock tokenizer
+        mock_tokenizer = Mock()
+        mock_tokenizer.pad_token_id = 0
+        mock_tokenizer.eos_token_id = 2
+        mock_tokenizer.bos_token_id = 1
+        mock_tokenizer.eos_token = "<|endoftext|>"
+        mock_tokenizer.apply_chat_template = Mock(return_value=[1, 10, 20, 30, 2])
+        mock_tokenizer.encode = Mock(return_value=[2])
+
+        return ChatTemplateStrategyWithKDv2(
+            prompter=mock_prompter,
+            tokenizer=mock_tokenizer,
+            train_on_inputs=False,
+            sequence_len=512,
+            logprobs_field="logprobs",
+            gen_temperature=1.0,
+            kd_temperature=1.0,
+        )
+
+    def test_v2_prepare_kd_fields_adds_target_token_ids(self, v2_strategy):
+        """
+        Test that v2's _prepare_kd_fields hook adds target_token_ids.
+
+        Validates the Template Method pattern fix where v2 overrides
+        the hook to add target_token_ids before transform.
+        """
+        tokenized = {"input_ids": [1, 10, 20, 30, 2], "labels": [1, 10, 20, 30, 2]}
+        original = {"target_token_ids": [[10, 20], [30, 40]]}
+
+        result = v2_strategy._prepare_kd_fields(tokenized, original)
+
+        assert "target_token_ids" in result
+        assert result["target_token_ids"] == [[10, 20], [30, 40]]
+
+    def test_v2_prepare_kd_fields_handles_missing_field(self, v2_strategy):
+        """Test hook handles missing target_token_ids gracefully"""
+        tokenized = {"input_ids": [1, 10, 20, 30, 2], "labels": [1, 10, 20, 30, 2]}
+        original = {}
+
+        result = v2_strategy._prepare_kd_fields(tokenized, original)
+
+        assert "target_token_ids" not in result
+
+    def test_v2_transform_requires_target_token_ids(self, v2_strategy):
+        """
+        Test v2's transform fails without target_token_ids.
+
+        Validates the bug fix - transform expects target_token_ids
+        to be added by the hook.
+        """
+        sample = {
+            "input_ids": [1, 10, 20, 30, 2],
+            "labels": [1, 10, 20, 30, 2],
+            "logprobs": [[-0.1, -0.2], [-0.3, -0.4]],
+        }
+
+        with pytest.raises(KeyError, match="target_token_ids"):
+            v2_strategy.transform_logprobs(sample)