Fix serve config parsing

[gvspraveen]

This seems to have been accidentally removed in previous PR
serve config.yaml expects args to be dictionary. See existing readme in ray-llm repo

Accidentally remove RouterArgs parsing is causing the following issue

pydantic.error_wrappers.ValidationError: 2 validation errors for VLLMApp
engine_config
  field required (type=value_error.missing)
scaling_config
  field required (type=value_error.missing)

Tested locally with following serve config and verified with fix i am able to start server

applications:
- name: meta-llama--Llama-2-7b-chat-hf
  route_prefix: /
  import_path: aviary.backend:router_application
  args:
    models:
      - "path to model"

Looks like others are also facing this: https://ray-distributed.slack.com/archives/C053J39MJ3A/p1696320039604519

[shrekris-anyscale]

This new typing doesn't support Dict[str, Union[str, LLMApp]] (the current typing). @Yard1 should we support that type?

[Yard1]

We don't have to anymore, no

[shrekris-anyscale]

Thanks for taking this @gvspraveen!