feat: add --providers to llama stack build (#1718)

# What does this PR do?

allow users to specify only the providers they want in the llama stack
build command. If a user wants a non-interactive build, but doesn't want
to use a template, `--providers` allows someone to specify something
like `--providers inference=remote::ollama` for a distro with JUST
ollama

## Test Plan

`llama stack build --providers inference=remote::ollama --image-type
venv`
<img width="1084" alt="Screenshot 2025-03-20 at 9 34 14 AM"
src="https://github.com/user-attachments/assets/502b5fa2-edab-4267-a595-4f987204a6a9"
/>

`llama stack run --image-type venv
/Users/charliedoern/projects/Documents/llama-stack/venv-run.yaml`
<img width="1149" alt="Screenshot 2025-03-20 at 9 35 19 AM"
src="https://github.com/user-attachments/assets/433765f3-6b7f-4383-9241-dad085b69228"
/>

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>
Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Sébastien Han <seb@redhat.com>

llama_stack/cli/stack/_build.py

@@ -89,6 +89,43 @@ def run_stack_build_command(args: argparse.Namespace) -> None:
                 color="red",
             )
             sys.exit(1)
+    elif args.providers:
+        providers = dict()
+        for api_provider in args.providers.split(","):
+            if "=" not in api_provider:
+                cprint(
+                    "Could not parse `--providers`. Please ensure the list is in the format api1=provider1,api2=provider2",
+                    color="red",
+                )
+                sys.exit(1)
+            api, provider = api_provider.split("=")
+            providers_for_api = get_provider_registry().get(Api(api), None)
+            if providers_for_api is None:
+                cprint(
+                    f"{api} is not a valid API.",
+                    color="red",
+                )
+                sys.exit(1)
+            if provider in providers_for_api:
+                providers.setdefault(api, []).append(provider)
+            else:
+                cprint(
+                    f"{provider} is not a valid provider for the {api} API.",
+                    color="red",
+                )
+                sys.exit(1)
+        distribution_spec = DistributionSpec(
+            providers=providers,
+            description=",".join(args.providers),
+        )
+        if not args.image_type:
+            cprint(
+                f"Please specify a image-type (container | conda | venv) for {args.template}",
+                color="red",
+            )
+            sys.exit(1)
+
+        build_config = BuildConfig(image_type=args.image_type, distribution_spec=distribution_spec)
     elif not args.config and not args.template:
         name = prompt(
             "> Enter a name for your Llama Stack (e.g. my-local-stack): ",

llama_stack/cli/stack/build.py

@@ -75,6 +75,12 @@ the build. If not specified, currently active environment will be used if found.
             default=False,
             help="Run the stack after building using the same image type, name, and other applicable arguments",
         )
+        self.parser.add_argument(
+            "--providers",
+            type=str,
+            default=None,
+            help="Build a config for a list of providers and only those providers. This list is formatted like: api1=provider1,api2=provider2. Where there can be multiple providers per API.",
+        )
 
     def _run_stack_build_command(self, args: argparse.Namespace) -> None:
         # always keep implementation completely silo-ed away from CLI so CLI