chore: add subcommands description in help (#1219)

# What does this PR do?
[Provide a short summary of what this PR does and why. Link to relevant
issues if applicable.]

```
before:
$ llama
usage: llama [-h] {model,stack,download,verify-download} ...

Welcome to the Llama CLI

options:
  -h, --help            show this help message and exit

subcommands:
  {model,stack,download,verify-download}

$ llama model --help
usage: llama model [-h] {download,list,prompt-format,describe,verify-download,remove} ...

Work with llama models

options:
  -h, --help            show this help message and exit

model_subcommands:
  {download,list,prompt-format,describe,verify-download,remove}

$ llama stack --help
usage: llama stack [-h] [--version] {build,list-apis,list-providers,run} ...

Operations for the Llama Stack / Distributions

options:
  -h, --help            show this help message and exit
  --version             show program's version number and exit

stack_subcommands:
  {build,list-apis,list-providers,run}

===================
after:
$ llama
usage: llama [-h] {model,stack,download,verify-download} ...

Welcome to the Llama CLI

options:
  -h, --help            show this help message and exit

subcommands:
  {model,stack,download,verify-download}

  model                 Work with llama models
  stack                 Operations for the Llama Stack / Distributions
  download              Download a model from llama.meta.com or Hugging Face Hub
  verify-download       Verify integrity of downloaded model files

$ llama model --help
usage: llama model [-h] {download,list,prompt-format,describe,verify-download,remove} ...

Work with llama models

options:
  -h, --help            show this help message and exit

model_subcommands:
  {download,list,prompt-format,describe,verify-download,remove}

  download              Download a model from llama.meta.com or Hugging Face Hub
  list                  Show available llama models
  prompt-format         Show llama model message formats
  describe              Show details about a llama model
  verify-download       Verify the downloaded checkpoints' checksums for models downloaded from Meta
  remove                Remove the downloaded llama model

$ llama stack --help
usage: llama stack [-h] [--version] {build,list-apis,list-providers,run} ...

Operations for the Llama Stack / Distributions

options:
  -h, --help            show this help message and exit
  --version             show program's version number and exit

stack_subcommands:
  {build,list-apis,list-providers,run}

  build                 Build a Llama stack container
  list-apis             List APIs part of the Llama Stack implementation
  list-providers        Show available Llama Stack Providers for an API
  run                   Start the server for a Llama Stack Distribution. You should have already built (or downloaded) and configured the distribution.
```

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

---------

Signed-off-by: reidliu <reid201711@gmail.com>
Co-authored-by: reidliu <reid201711@gmail.com>

llama_stack/cli/stack/utils.py

@@ -0,0 +1,14 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+def print_subcommand_description(parser, subparsers):
+    """Print descriptions of subcommands."""
+    description_text = ""
+    for name, subcommand in subparsers.choices.items():
+        description = subcommand.description
+        description_text += f"  {name:<21} {description}\n"
+    parser.epilog = description_text