Add "from_draft" to GenerationResponse (#1272)

* Add from_draft field in GenerationResponse

* Cleanup

* Re-work for minimal changes, add test

* Fix comment

llms/tests/test_generate.py

@@ -1,17 +1,24 @@
 # Copyright © 2024 Apple Inc.
 
 import unittest
+from typing import List
 
 from mlx_lm.sample_utils import make_logits_processors
-from mlx_lm.utils import generate, load
+from mlx_lm.utils import (
+    GenerationResponse,
+    generate,
+    load,
+    make_sampler,
+    stream_generate,
+)
 
 
 class TestGenerate(unittest.TestCase):
 
     @classmethod
     def setUpClass(cls):
-        HF_MODEL_PATH = "mlx-community/Qwen1.5-0.5B-Chat-4bit"
-        cls.model, cls.tokenizer = load(HF_MODEL_PATH)
+        cls.HF_MODEL_PATH = "mlx-community/Qwen1.5-0.5B-Chat-4bit"
+        cls.model, cls.tokenizer = load(cls.HF_MODEL_PATH)
 
     def test_generate(self):
         # Simple test that generation runs
@@ -51,6 +58,34 @@ class TestGenerate(unittest.TestCase):
         )
         self.assertEqual(len(all_toks), len(init_toks) + 5)
 
+    def test_stream_generate_speculative(self):
+        # Use same model as draft model, this is not a speed test
+        draft_model, _ = load(self.HF_MODEL_PATH)
+
+        results: List[GenerationResponse] = []
+        drafted: List[bool] = []
+
+        # make a determinate sampler
+        sampler = make_sampler(temp=0.0)
+
+        for generation_result in stream_generate(
+            model=self.model,
+            tokenizer=self.tokenizer,
+            prompt="hello",
+            max_tokens=5,
+            draft_model=draft_model,
+            num_draft_tokens=2,
+            sampler=sampler,
+        ):
+            drafted.append(generation_result.from_draft)
+            results.append(generation_result)
+
+        self.assertEqual(len(results), 5)
+        # since num_draft_tokens is 2 and draft model is the same, the
+        # first 2 generations should be drafts, the third should come
+        # from the target model, and last two should be drafts
+        self.assertEqual(drafted, [True, True, False, True, True])
+
 
 if __name__ == "__main__":
     unittest.main()