[post training] define llama stack post training dataset format (#717)

## context
In this PR, we defined 2 llama stack dataset formats (instruct, dialog)

- For instruct dataset format, the column schema will be
[chat_completion_input, expected_answer], which is consistent with the
eval data format. This dataset format is the abstract of single turn QA
style post training data
- For dialog dataset format, the column schema will be [dialog], which
is a list of user messages and assistant messages that interleave
together. During training, the whole list will be the model input and
the loss is calculated on assistant messages only. This dataset format
is the abstract of multi turn chat style post training data

## changes
- defined the 2 llama stack dataset formats
- an adapter to convert llama stack dataset format to torchtune dataset
format
- move dataset format validation to post training level instead of
torchtune level since it's not specific to torchtune
- add localfs as datasetio provider


## test 
instruct format
- use https://huggingface.co/datasets/llamastack/evals as dataset and
the training works as expected
<img width="1443" alt="Screenshot 2025-01-09 at 5 15 14 PM"
src="https://github.com/user-attachments/assets/2c37a936-c67a-4726-90e0-23fa0ba7000f"
/>

- use my generated local dataset and the training works as expected

<img width="1617" alt="Screenshot 2025-01-09 at 5 19 11 PM"
src="https://github.com/user-attachments/assets/0bdccbbf-bac2-472a-a365-15213e49bbfa"
/>


dialog format
- use my generated local dataset and the training works as expected
<img width="1588" alt="Screenshot 2025-01-09 at 5 23 16 PM"
src="https://github.com/user-attachments/assets/893915ba-41a3-4d51-948b-e872060ecede"
/>

llama_stack/apis/common/type_system.py

@@ -54,6 +54,12 @@ class AgentTurnInputType(BaseModel):
     type: Literal["agent_turn_input"] = "agent_turn_input"
 
 
+class DialogType(BaseModel):
+    # expects List[Message] for messages
+    # this type semantically contains the output label whereas ChatCompletionInputType does not
+    type: Literal["dialog"] = "dialog"
+
+
 ParamType = register_schema(
     Annotated[
         Union[