Add doc for how to customize an LLM Tool. (#852)

# Description

Add doc for how to customize an LLM Tool.

# All Promptflow Contribution checklist:
- [X] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [X] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [X] Title of the pull request is clear and informative.
- [X] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

docs/how-to-guides/develop-a-tool/create-and-use-tool-package.md

@@ -165,3 +165,4 @@ Alternatively, you can test your tool package using the script below to ensure t
 [Customize your tool icon](add-a-tool-icon.md)  
 [Add category and tags for tool](add-category-and-tags-for-tool.md)  
 [Use file path as tool input](use-file-path-as-tool-input.md)  
+[Customize an LLM Tool](customize_an_llm_tool.md)  

docs/how-to-guides/develop-a-tool/customize_an_llm_tool.md

@@ -0,0 +1,60 @@
+# Customize an LLM Tool
+In this document, we will guide you through the process of customizing an LLM tool, allowing users to seamlessly connect to a large language model with prompt tuning experience using a `PromptTemplate`.
+
+## Prerequisites
+- Please ensure that your [Prompt flow for VS Code](https://marketplace.visualstudio.com/items?itemName=prompt-flow.prompt-flow) is updated to version 1.2.0 or later.
+
+## How to customize an LLM tool
+Here we use [an existing tool package](https://github.com/microsoft/promptflow/tree/main/examples/tools/tool-package-quickstart/my_tool_package) as an example. If you want to create your own tool, please refer to [create and use tool package](create-and-use-tool-package.md).  
+
+1. Develop the tool code as in [this example](https://github.com/microsoft/promptflow/blob/main/examples/tools/tool-package-quickstart/my_tool_package/tools/tool_with_custom_llm_type.py).
+- Add a `CustomConnection` input to the tool, which is used to authenticate and establish a connection to the large language model.
+- Add a `PromptTemplate` input to the tool, which serves as an argument to be passed into the large language model.
+
+    ```python
+    from jinja2 import Template
+    from promptflow import tool
+    from promptflow.connections import CustomConnection
+    from promptflow.contracts.types import PromptTemplate
+
+
+    @tool
+    def my_tool(connection: CustomConnection, prompt: PromptTemplate, **kwargs) -> str:
+        # Customize your own code to use the connection and prompt here.
+        rendered_prompt = Template(prompt, trim_blocks=True, keep_trailing_newline=True).render(**kwargs)
+        return rendered_prompt
+    ```
+
+2. Generate the custom LLM tool YAML.  
+    Run the command below in your tool project directory to automatically generate your tool YAML, use _-t "custom_llm"_ or _--tool-type "custom_llm"_ to indicate this is a custom LLM tool:  
+    ```
+    python <path-to-scripts>\tool\generate_package_tool_meta.py -m <tool_module> -o <tool_yaml_path> -t "custom_llm"
+    ```
+    Here we use [an existing tool](https://github.com/microsoft/promptflow/blob/main/examples/tools/tool-package-quickstart/my_tool_package/yamls/tool_with_custom_llm_type.yaml) as an example.
+    ```
+    cd D:\proj\github\promptflow\examples\tools\tool-package-quickstart
+
+    python D:\proj\github\promptflow\scripts\tool\generate_package_tool_meta.py -m my_tool_package.tools.tool_with_custom_llm_type -o my_tool_package\yamls\tool_with_custom_llm_type.yaml -n "My Custom LLM Tool" -d "This is a tool to demonstrate how to customize an LLM tool with a PromptTemplate." -t "custom_llm"
+    ```
+    This command will generate a YAML file as follows:
+
+    ```yaml
+    my_tool_package.tools.tool_with_custom_llm_type.my_tool:
+    name: My Custom LLM Tool
+    description: This is a tool to demonstrate how to customize an LLM tool with a PromptTemplate.
+    # The type is custom_llm.
+    type: custom_llm
+    module: my_tool_package.tools.tool_with_custom_llm_type
+    function: my_tool
+    inputs:
+        connection:
+        type:
+            - CustomConnection
+    ```
+
+## Use the tool in VS Code
+Follow the steps to [build and install your tool package](create-and-use-tool-package.md#build-and-share-the-tool-package) and [use your tool from VS Code extension](create-and-use-tool-package.md#use-your-tool-from-vscode-extension).  
+
+Here we use an existing flow to demonstrate the experience, open [this flow](https://github.com/microsoft/promptflow/blob/main/examples/tools/use-cases/custom_llm_tool_showcase/flow.dag.yaml) in VS Code extension.  
+- There is a node named "my_custom_llm_tool" with a prompt template file. You can either use an existing file or create a new one as the prompt template file.  
+![use_my_custom_llm_tool](../../media/how-to-guides/develop-a-tool/use_my_custom_llm_tool.png)

docs/how-to-guides/develop-a-tool/index.md

@@ -9,4 +9,5 @@ create-and-use-tool-package
 add-a-tool-icon
 add-category-and-tags-for-tool
 use-file-path-as-tool-input
+customize_an_llm_tool
 ```

examples/README.md

@@ -81,7 +81,7 @@
 | [cascading-inputs-tool-showcase](tools/use-cases/cascading-inputs-tool-showcase/README.md) | [![samples_tools_use_cases_cascading_inputs_tool_showcase](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_cascading_inputs_tool_showcase.yml/badge.svg?branch=main)](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_cascading_inputs_tool_showcase.yml) | This is a flow demonstrating the use of a tool with cascading inputs which frequently used in situations where the selection in one input field determines what subsequent inputs should be shown, and it helps in creating a more efficient, user-friendly, and error-free input process |
 | [custom-strong-type-connection-package-tool-showcase](tools/use-cases/custom-strong-type-connection-package-tool-showcase/README.md) | [![samples_tools_use_cases_custom_strong_type_connection_package_tool_showcase](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_strong_type_connection_package_tool_showcase.yml/badge.svg?branch=main)](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_strong_type_connection_package_tool_showcase.yml) |  This is a flow demonstrating the use of a package tool with custom string type connection which provides a secure way to manage credentials for external APIs and data sources, and it offers an improved user-friendly and intellisense experience compared to custom connections |
 | [custom-strong-type-connection-script-tool-showcase](tools/use-cases/custom-strong-type-connection-script-tool-showcase/README.md) | [![samples_tools_use_cases_custom_strong_type_connection_script_tool_showcase](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_strong_type_connection_script_tool_showcase.yml/badge.svg?branch=main)](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_strong_type_connection_script_tool_showcase.yml) |  This is a flow demonstrating the use of a script tool with custom string type connection which provides a secure way to manage credentials for external APIs and data sources, and it offers an improved user-friendly and intellisense experience compared to custom connections |
-| [custom_llm_tool_showcase](tools/use-cases/custom_llm_tool_showcase/README.md) | [![samples_tools_use_cases_custom_llm_tool_showcase](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_llm_tool_showcase.yml/badge.svg?branch=main)](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_llm_tool_showcase.yml) |  This is a flow demonstrating how to use a `custom_llm` tool, `custom_llm` tool allows users to connect to a customized large language model which is fine-tuned by tool developer |
+| [custom_llm_tool_showcase](tools/use-cases/custom_llm_tool_showcase/README.md) | [![samples_tools_use_cases_custom_llm_tool_showcase](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_llm_tool_showcase.yml/badge.svg?branch=main)](https://github.com/microsoft/promptflow/actions/workflows/samples_tools_use_cases_custom_llm_tool_showcase.yml) | This is a flow demonstrating how to use a `custom_llm` tool, which enables users to seamlessly connect to a large language model with prompt tuning experience using a `PromptTemplate` |
 
 
 ### Connections ([connections](connections))

examples/tools/tool-package-quickstart/my_tool_package/yamls/tool_with_custom_llm_type.yaml

@@ -1,6 +1,6 @@
 my_tool_package.tools.tool_with_custom_llm_type.my_tool:
   name: My Custom LLM Tool
-  description: This is a tool to demonstrate the custom_llm tool type
+  description: This is a tool to demonstrate how to customize an LLM tool with a PromptTemplate.
   type: custom_llm
   module: my_tool_package.tools.tool_with_custom_llm_type
   function: my_tool

examples/tools/use-cases/custom_llm_tool_showcase/README.md

@@ -1,5 +1,5 @@
 # Flow with custom_llm tool
-This is a flow demonstrating how to use a `custom_llm` tool, `custom_llm` tool allows users to connect to a customized large language model which is fine-tuned by tool developer. 
+This is a flow demonstrating how to use a `custom_llm` tool, which enables users to seamlessly connect to a large language model with prompt tuning experience using a `PromptTemplate`.
 
 Tools used in this flow:
 - `custom_llm` Tool

examples/tools/use-cases/custom_llm_tool_showcase/flow.dag.yaml

@@ -1,4 +1,10 @@
-inputs: {}
+inputs:
+  website_name:
+    type: string
+    default: Microsoft
+  user_name:
+    type: string
+    default: ""
 outputs:
   output:
     type: string
@@ -12,3 +18,5 @@ nodes:
     path: prompt_template.jinja2
   inputs:
     connection: basic_custom_connection
+    website_name: ${inputs.website_name}
+    user_name: ${inputs.user_name}

examples/tools/use-cases/custom_llm_tool_showcase/prompt_template.jinja2

@@ -1 +1,6 @@
-This is the prompt for my custom LLM tool.
+Welcome to {{ website_name }}!
+{% if user_name %}
+    Hello, {{ user_name }}!
+{% else %}
+    Hello there!
+{% endif %}