chore: add subcommands description in help

[reidliu41]

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.

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.]

[leseb]

Can we avoid duplicating print_subcommand_description each time?