Fix args type (huggingface#1214)

* fix args type

* add args desc

trl/trainer/utils.py

@@ -61,11 +61,11 @@ class DataCollatorForCompletionOnlyLM(DataCollatorForLanguageModeling):
     calculated on the completion made by the assistant.
 
     Args:
-        instruction_template (`Optional[str]`): the template form that indicates the start of the human instruction, typically something like
-            '### Human:\n'. Useful for assistant-style conversation datasets
         response_template (`Union[str, List[int]]`): the template form that indicates the start of the response, typically something like
             '### Response:\n'. It can also be passed as tokenized ids, which can be useful when using a tokenizer that encodes the response
             differently if it does not have proper context.
+        instruction_template (`Union[str, List[int]]`): the template form that indicates the start of the human instruction, typically something like
+            '### Human:\n'. Useful for assistant-style conversation datasets. It can also be passed as tokenized ids.
         mlm (`bool`, *optional*, defaults to `False`): Whether or not to use masked language modeling in the underlying
             `DataCollatorForLanguageModeling` class. Note that this option currently has no effect but is present
              for flexibility and backwards-compatibility.