Allow explicit model alias registration

[ehhuang]

• Currently model_id in run config is only used as the identifier if it does not equal provider_model_id.
• If model_id == provider_model_id, the identifier becomes provider_id/provider_model_id.
• This is confusing behavior since the user-specified model_id does not get registered at all.
• Furthermore, currently it's actually not possible to register an alias that's equal to provider_model_id (always becomes provider_id/provider_model_id.

Proposal:

1. Make provider_model_id required: this references the provider model that's being registered.
2. model_id becomes an optional parameter:
   a. if unspecified, identifier = provider_id/provider_model_id
   b. if specified, identifier = model_id

Note that this is not backward compatible, since existing run configs can have multiple entries with the same model_id.

[ashwinb]

Yes this is not backward compatible, but the behavior makes sense to me

[mansiUnifynd]

I’ve been trying to register Anthropic models, but every time I add one it says the model isn’t supported.
Could someone please guide me on how to properly register Anthropic models on Llama Stack? It would be a great help.
Heres the list of things I have done:

1. I have update the API Key in my run.yaml
   - provider_id: anthropic
   provider_type: remote::anthropic
   config:
   api_key: ${env.ANTHROPIC_API_KEY:""}

2. Below is the command that i am using to register the model:
   llama-stack-client models register claude-3-5-sonnet --provider-id anthropic --provider-model-id claude-3-5-sonnet-latest --metadata {} --model-type llm

ERROR I AM GETTING:
INFO:httpx:HTTP Request: POST http://localhost:8321/v1/models "HTTP/1.1 400 Bad Request"
╭─────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ Failed to register model │
│ │
│ Error Type: BadRequestError │
│ Details: Error code: 400 - {'detail': 'Invalid value: Unsupported model: claude-3-5-sonnet-latest'} │