Add ability to fetch raw prompt and completion text from completion datasets

llms/mlx_lm/tuner/datasets.py

@@ -84,12 +84,15 @@ class CompletionsDataset:
     def __len__(self):
         return len(self._data)
 
+    def get_prompt_and_completion(self, idx: int):
+        return self._data[idx][self._prompt_key], self._data[idx][self._completion_key]
+
 
 class CompletionsDatasetCollection:
     def __init__(self, data: List[Union[ChatDataset, CompletionsDataset]]):
         self.collection = data
 
-    def __getitem__(self, idx: int):
+    def __fetch_and_process_item__(self, idx: int, handler_fn: Callable):
         iteration = iter(self.collection)
         item = next(iteration)
 
@@ -98,13 +101,25 @@ class CompletionsDatasetCollection:
         while True:
             try:
                 if (curr_idx + 1) <= len(item):
-                    return item[curr_idx]
+                    return handler_fn(item, curr_idx)
                 else:
                     curr_idx -= len(item)
                     item = next(iteration)
             except StopIteration:
                 raise IndexError(idx)
 
+    def __getitem__(self, idx: int):
+        def getitem(dataset: CompletionsDataset, index: int):
+            return dataset[index]
+
+        return self.__fetch_and_process_item__(idx, getitem)
+
+    def get_prompt_and_completion(self, idx: int):
+        def getitem(dataset: CompletionsDataset, index: int):
+            dataset.get_prompt_and_completion(index)
+
+        return self.__fetch_and_process_item__(idx, getitem)
+
     def __len__(self):
         return sum(map(len, self.collection))