text: Title: [Feature]: specify model only in config.yaml
  Body: ### 🚀 The feature, motivation and pitch
  
  It would be nice not to need anything but a config file to start `serve`:
  
  ```sh
  vllm serve --config scenario.yaml
  ```
  
  Right now, the `model_tag` appears to be required via the CLI.
  
  Why not allow this via a config file too? For example:
  
  ```yaml
  model: Qwen/Qwen2.5-Coder-7B
  # ... args
  ```
  
  Furthermore, the web based docs for `vllm serve` don't list the `model_tag` as required: https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#vllm-serve and instead suggest I can use `--model` and thus based on the [config section](https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#configuration-file) I assume that would map to `model: Qwen/Qwen2.5-Coder-7B`?
  
  This would be a great way to encapsulate everything I need for a particular serve "environment" into a config file to quickly start different scenarios.
  
  ### Alternatives
  
  _No response_
  
  ### Additional context
  
  _No response_
  
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