Chat Template support for function calling and RAG

docs/source/en/chat_templating.md

@@ -233,6 +233,332 @@ The sun.</s>
 
 From here, just continue training like you would with a standard language modelling task, using the `formatted_chat` column.
 
+## Advanced: Extra inputs to chat templates
+
+The only argument that `apply_chat_template` requires is `messages`. However, you can pass any keyword
+argument to `apply_chat_template` and it will be accessible inside the template. This gives you a lot of freedom to use
+chat templates for many things. There are no restrictions on the names or the format of these arguments - you can pass
+strings, lists, dicts or whatever else you want. 
+
+That said, there are some common use-cases for these extra arguments,
+such as passing tools for function calling, or documents for retrieval-augmented generation. In these common cases,
+we have some opinionated recommendations about what the names and formats of these arguments should be, which are
+described in the sections below. We encourage model authors to make their chat templates compatible with this format,
+to make it easy to transfer tool-calling code between models.
+
+## Advanced: Tool use / function calling
+
+"Tool use" LLMs can choose to call functions as external tools before generating an answer. When passing tools
+to a tool-use model, you can simply pass a list of functions to the `tools` argument:
+
+```python
+import datetime
+
+def current_time():
+    """Get the current local time as a string."""
+    return str(datetime.now())
+
+def multiply(a: float, b: float):
+    """
+    A function that multiplies two numbers
+    
+    Args:
+        a: The first number to multiply
+        b: The second number to multiply
+    """
+    return a * b
+
+tools = [current_time, multiply]
+
+model_input = tokenizer.apply_chat_template(
+    messages,
+    tools=tools
+)
+```
+
+In order for this to work correctly, you should write your functions in the format above, so that they can be parsed
+correctly as tools. Specifically, you should follow these rules:
+
+- The function should have a descriptive name
+- Every argument must have a type hint
+- The function must have a docstring in the standard Google style (in other words, an initial function description  
+  followed by an `Args:` block that describes the arguments, unless the function does not have any arguments. 
+- Do not include types in the `Args:` block. In other words, write `a: The first number to multiply`, not
+  `a (int): The first number to multiply`. Type hints should go in the function header instead.
+- The function can have a return type and a `Returns:` block in the docstring. However, these are optional
+  because most tool-use models ignore them.
+
+### Passing tool results to the model
+
+The sample code above is enough to list the available tools for your model, but what happens if it wants to actually use
+one? If that happens, you should:
+
+1. Parse the model's output to get the tool name(s) and arguments.
+2. Add the model's tool call(s) to the conversation.
+3. Call the corresponding function(s) with those arguments.
+4. Add the result(s) to the conversation
+
+### A complete tool use example
+
+Let's walk through a tool use example, step by step. For this example, we will use an 8B `Hermes-2-Pro` model,
+as it is one of the highest-performing tool-use models in its size category at the time of writing. If you have the
+memory, you can consider using a larger model instead like [Command-R](https://huggingface.co/CohereForAI/c4ai-command-r-v01)
+or [Mixtral-8x22B](https://huggingface.co/mistralai/Mixtral-8x22B-Instruct-v0.1), both of which also support tool use
+and offer even stronger performance.
+
+First, let's load our model and tokenizer:
+
+```python
+import torch
+from transformers import AutoModelForCausalLM, AutoTokenizer
+
+checkpoint = "NousResearch/Hermes-2-Pro-Llama-3-8B"
+
+tokenizer = AutoTokenizer.from_pretrained(checkpoint, revision="pr/13")
+model = AutoModelForCausalLM.from_pretrained(checkpoint, torch_dtype=torch.bfloat16, device_map="auto")
+```
+
+Next, let's define a list of tools:
+
+```python
+def get_current_temperature(location: str, unit: str) -> float:
+    """
+    Get the current temperature at a location.
+    
+    Args:
+        location: The location to get the temperature for, in the format "City, Country"
+        unit: The unit to return the temperature in. (choices: ["celsius", "fahrenheit"])
+    Returns:
+        The current temperature at the specified location in the specified units, as a float.
+    """
+    return 22.  # A real function should probably actually get the temperature!
+
+def get_current_wind_speed(location: str) -> float:
+    """
+    Get the current wind speed in km/h at a given location.
+    
+    Args:
+        location: The location to get the temperature for, in the format "City, Country"
+    Returns:
+        The current wind speed at the given location in km/h, as a float.
+    """
+    return 6.  # A real function should probably actually get the wind speed!
+
+tools = [get_current_temperature, get_current_wind_speed]
+```
+
+Now, let's set up a conversation for our bot:
+
+```python
+messages = [
+  {"role": "system", "content": "You are a bot that responds to weather queries. You should reply with the unit used in the queried location."},
+  {"role": "user", "content": "Hey, what's the temperature in Paris right now?"}
+]
+```
+
+Now, let's apply the chat template and generate a response:
+
+```python
+inputs = tokenizer.apply_chat_template(messages, chat_template="tool_use", tools=tools, add_generation_prompt=True, return_dict=True, return_tensors="pt")
+inputs = {k: v.to(model.device) for k, v in inputs.items()}
+out = model.generate(**inputs, max_new_tokens=128)
+print(tokenizer.decode(out[0][len(inputs["input_ids"][0]):]))
+```
+
+And we get:
+
+```text
+<tool_call>
+{"arguments": {"location": "Paris, France", "unit": "celsius"}, "name": "get_current_temperature"}
+</tool_call><|im_end|>
+```
+
+The model has called the function with valid arguments, in the format requested by the function docstring. It has
+inferred that we're most likely referring to the Paris in France, and it remembered that, as the home of SI units,
+the temperature in France should certainly be displayed in Celsius.
+
+Let's append the model's tool call to the conversation. Note that we generate a random `tool_call_id` here. These IDs
+are not used by all models, but they allow models to issue multiple tool calls at once and keep track of which response
+corresponds to which call. You can generate them any way you like, but they should be unique within each chat.
+
+```python
+tool_call_id = "vAHdf3"  # Random ID, should be unique for each tool call
+tool_call = {"name": "get_current_temperature", "arguments": {"location": "Paris, France", "unit": "celsius"}}
+messages.append({"role": "assistant", "tool_calls": [{"id": tool_call_id, "type": "function", "function": tool_call}]})
+```
+
+
+Now that we've added the tool call to the conversation, we can call the function and append the result to the
+conversation. Since we're just using a dummy function for this example that always returns 22.0, we can just append 
+that result directly. Again, note the `tool_call_id` - this should match the ID used in the tool call above.
+
+```python
+messages.append({"role": "tool", "tool_call_id": tool_call_id, "name": "get_current_temperature", "content": "22.0"})
+```
+
+Finally, let's let the assistant read the function outputs and continue chatting with the user:
+
+```python
+inputs = tokenizer.apply_chat_template(messages, chat_template="tool_use", tools=tools, add_generation_prompt=True, return_dict=True, return_tensors="pt")
+inputs = {k: v.to(model.device) for k, v in inputs.items()}
+out = model.generate(**inputs, max_new_tokens=128)
+print(tokenizer.decode(out[0][len(inputs["input_ids"][0]):]))
+```
+
+And we get:
+
+```text
+The current temperature in Paris, France is 22.0 ° Celsius.<|im_end|>
+```
+
+Although this was a simple demo with dummy tools and a single call, the same technique works with 
+multiple real tools and longer conversations. This can be a powerful way to extend the capabilities of conversational
+agents with real-time information, computational tools like calculators, or access to large databases.
+
+<Tip>
+Not all of the tool-calling features shown above are used by all models. Some use tool call IDs, others simply use the function name and
+match tool calls to results using the ordering, and there are several models that use neither and only issue one tool 
+call at a time to avoid confusion. If you want your code to be compatible across as many models as possible, we 
+recommend structuring your tools calls like we've shown here, and returning tool results in the order that
+they were issued by the model. The chat templates on each model should handle the rest.
+</Tip>
+
+### Understanding tool schemas
+
+Each function you pass to the `tools` argument of `apply_chat_template` is converted into a 
+[JSON schema](https://json-schema.org/learn/getting-started-step-by-step). These schemas
+are then passed to the model chat template. In other words, tool-use models do not see your functions directly, and they
+never see the actual code inside them. What they care about is the function **definitions** and the **arguments** they
+need to pass to them - they care about what the tools do and how to use them, not how they work! It is up to you
+to read their outputs, detect if they have requested to use a tool, pass their arguments to the tool function, and
+return the response in the chat.
+
+Generating JSON schemas to pass to the template should be automatic and invisible as long as your functions
+follow the specification above, but if you encounter problems, or you simply want more control over the conversion, 
+you can handle the conversion manually. Here is an example of a manual schema conversion.
+
+```python
+from transformers.utils import get_json_schema
+
+def multiply(a: float, b: float):
+    """
+    A function that multiplies two numbers
+    
+    Args:
+        a: The first number to multiply
+        b: The second number to multiply
+    """
+    return a * b
+
+schema = get_json_schema(multiply)
+print(schema)
+```
+
+This will yield:
+
+```json
+{
+  "type": "function", 
+  "function": {
+    "name": "multiply", 
+    "description": "A function that multiplies two numbers", 
+    "parameters": {
+      "type": "object", 
+      "properties": {
+        "a": {
+          "type": "number", 
+          "description": "The first number to multiply"
+        }, 
+        "b": {
+          "type": "number",
+          "description": "The second number to multiply"
+        }
+      }, 
+      "required": ["a", "b"]
+    }
+  }
+}
+```
+
+If you wish, you can edit these schemas, or even write them from scratch yourself without using `get_json_schema` at 
+all. JSON schemas can be passed directly to the `tools` argument of 
+`apply_chat_template` - this gives you a lot of power to define precise schemas for more complex functions. Be careful,
+though - the more complex your schemas, the more likely the model is to get confused when dealing with them! We 
+recommend simple function signatures where possible, keeping arguments (and especially complex, nested arguments) 
+to a minimum.
+
+Here is an example of defining schemas by hand, and passing them directly to `apply_chat_template`:
+
+```python
+# A simple function that takes no arguments
+current_time = {
+  "type": "function", 
+  "function": {
+    "name": "current_time",
+    "description": "Get the current local time as a string.",
+    "parameters": {
+      'type': 'object',
+      'properties': {}
+    }
+  }
+}
+
+# A more complete function that takes two numerical arguments
+multiply = {
+  'type': 'function',
+  'function': {
+    'name': 'multiply',
+    'description': 'A function that multiplies two numbers', 
+    'parameters': {
+      'type': 'object', 
+      'properties': {
+        'a': {
+          'type': 'number',
+          'description': 'The first number to multiply'
+        }, 
+        'b': {
+          'type': 'number', 'description': 'The second number to multiply'
+        }
+      }, 
+      'required': ['a', 'b']
+    }
+  }
+}
+
+model_input = tokenizer.apply_chat_template(
+    messages,
+    tools = [current_time, multiply]
+)
+```
+
+## Advanced: Retrieval-augmented generation
+
+"Retrieval-augmented generation" or "RAG" LLMs can search a corpus of documents for information before responding
+to a query. This allows models to vastly expand their knowledge base beyond their limited context size. Our 
+recommendation for RAG models is that their template
+should accept a `documents` argument. This should be a list of documents, where each "document"
+is a single dict with `title` and `contents` keys, both of which are strings. Because this format is much simpler
+than the JSON schemas used for tools, no helper functions are necessary.
+
+Here's an example of a RAG template in action:
+
+```python
+document1 = {
+    "title": "The Moon: Our Age-Old Foe",
+    "contents": "Man has always dreamed of destroying the moon. In this essay, I shall..."
+}
+
+document2 = {
+    "title": "The Sun: Our Age-Old Friend",
+    "contents": "Although often underappreciated, the sun provides several notable benefits..."
+}
+
+model_input = tokenizer.apply_chat_template(
+    messages,
+    documents=[document1, document2]
+)
+```
+
 ## Advanced: How do chat templates work?
 
 The chat template for a model is stored on the `tokenizer.chat_template` attribute. If no chat template is set, the