Update PythonListCustomToolGenerator to support overriding system prompt

Summary:

Support user supplied prompt template in PythonListCustomToolGenerator. This is to allow user to provided their own system prompt without having to format function descirptions.

Test Plan:
python -m unittest llama_models.llama3.tests.prompt_templates.test_system_prompts

models/llama3/prompt_templates/system_prompts.py

@@ -7,7 +7,7 @@
 
 import textwrap
 from datetime import datetime
-from typing import Any, List
+from typing import Any, List, Optional
 
 from llama_models.llama3.api.datatypes import (
     BuiltinTool,
@@ -215,14 +215,33 @@ def data_examples(self) -> List[List[ToolDefinition]]:
 
 
 class PythonListCustomToolGenerator(PromptTemplateGeneratorBase):  # noqa: N801
-    def gen(self, custom_tools: List[ToolDefinition]) -> PromptTemplate:
+    DEFAULT_PROMPT = textwrap.dedent(
+        """
+        You are an expert in composing functions. You are given a question and a set of possible functions.
+        Based on the question, you will need to make one or more function/tool calls to achieve the purpose.
+        If none of the function can be used, point it out. If the given question lacks the parameters required by the function,
+        also point it out. You should only return the function call in tools call sections.
+
+        {{ function_description }}
+        """.strip(
+            "\n"
+        )
+    )
+
+    def gen(
+        self, custom_tools: List[ToolDefinition], system_prompt: Optional[str] = None
+    ) -> PromptTemplate:
+        system_prompt = system_prompt or self.DEFAULT_PROMPT
+        return PromptTemplate(
+            system_prompt,
+            {"function_description": self._gen_function_description(custom_tools)},
+        )
+
+    def _gen_function_description(
+        self, custom_tools: List[ToolDefinition]
+    ) -> PromptTemplate:
         template_str = textwrap.dedent(
             """
-            You are an expert in composing functions. You are given a question and a set of possible functions.
-            Based on the question, you will need to make one or more function/tool calls to achieve the purpose.
-            If none of the function can be used, point it out. If the given question lacks the parameters required by the function,
-            also point it out. You should only return the function call in tools call sections.
-
             If you decide to invoke any of the function(s), you MUST put it in the format of [func_name1(params_name1=params_value1, params_name2=params_value2...), func_name2(params)]
             You SHOULD NOT include any other text in the response.
 
@@ -263,7 +282,7 @@ def gen(self, custom_tools: List[ToolDefinition]) -> PromptTemplate:
         return PromptTemplate(
             template_str.strip("\n"),
             {"tools": [t.model_dump() for t in custom_tools]},
-        )
+        ).render()
 
     def data_examples(self) -> List[List[ToolDefinition]]:
         return [

models/llama3/tests/prompt_templates/test_system_prompts.py

@@ -145,3 +145,49 @@ def test_llama_3_2_system_zero_shot(self):
             """
         )
         self.check_generator_output(generator, expected_text.strip("\n"))
+
+    def test_llama_3_2_provided_system_prompt(self):
+        generator = PythonListCustomToolGenerator()
+        expected_text = textwrap.dedent(
+            """
+            Overriding message.
+
+            If you decide to invoke any of the function(s), you MUST put it in the format of [func_name1(params_name1=params_value1, params_name2=params_value2...), func_name2(params)]
+            You SHOULD NOT include any other text in the response.
+
+            Here is a list of functions in JSON format that you can invoke.
+
+            [
+                {
+                    "name": "get_weather",
+                    "description": "Get weather info for places",
+                    "parameters": {
+                        "type": "dict",
+                        "required": ["city"],
+                        "properties": {
+                            "city": {
+                                "type": "string",
+                                "description": "The name of the city to get the weather for"
+                            },
+                            "metric": {
+                                "type": "string",
+                                "description": "The metric for weather. Options are: celsius, fahrenheit",
+                                "default": "celsius"
+                            }
+                        }
+                    }
+                }
+            ]"""
+        )
+        user_system_prompt = textwrap.dedent(
+            """
+            Overriding message.
+            
+            {{ function_description }}
+            """
+        )
+        example = generator.data_examples()[0]
+
+        pt = generator.gen(example, user_system_prompt)
+        text = pt.render()
+        assert text == expected_text, f"Expected:\n{expected_text}\nActual:\n{text}"