feat: Add Google Vertex AI inference provider support (#2841)

# What does this PR do?
- Add new Vertex AI remote inference provider with litellm integration
- Support for Gemini models through Google Cloud Vertex AI platform
- Uses Google Cloud Application Default Credentials (ADC) for
authentication
- Added VertexAI models: gemini-2.5-flash, gemini-2.5-pro,
gemini-2.0-flash.
- Updated provider registry to include vertexai provider
- Updated starter template to support Vertex AI configuration
- Added comprehensive documentation and sample configuration

<!-- If resolving an issue, uncomment and update the line below -->
relates to https://github.com/meta-llama/llama-stack/issues/2747

## 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.* -->

Signed-off-by: Eran Cohen <eranco@redhat.com>
Co-authored-by: Francisco Arceo <arceofrancisco@gmail.com>

llama_stack/distributions/ci-tests/build.yaml

@@ -14,6 +14,7 @@ distribution_spec:
     - provider_type: remote::openai
     - provider_type: remote::anthropic
     - provider_type: remote::gemini
+    - provider_type: remote::vertexai
     - provider_type: remote::groq
     - provider_type: remote::sambanova
     - provider_type: inline::sentence-transformers

llama_stack/distributions/ci-tests/run.yaml

@@ -65,6 +65,11 @@ providers:
     provider_type: remote::gemini
     config:
       api_key: ${env.GEMINI_API_KEY:=}
+  - provider_id: ${env.VERTEX_AI_PROJECT:+vertexai}
+    provider_type: remote::vertexai
+    config:
+      project: ${env.VERTEX_AI_PROJECT:=}
+      location: ${env.VERTEX_AI_LOCATION:=us-central1}
   - provider_id: groq
     provider_type: remote::groq
     config:

llama_stack/distributions/starter/build.yaml

@@ -14,6 +14,7 @@ distribution_spec:
     - provider_type: remote::openai
     - provider_type: remote::anthropic
     - provider_type: remote::gemini
+    - provider_type: remote::vertexai
     - provider_type: remote::groq
     - provider_type: remote::sambanova
     - provider_type: inline::sentence-transformers

llama_stack/distributions/starter/run.yaml

@@ -65,6 +65,11 @@ providers:
     provider_type: remote::gemini
     config:
       api_key: ${env.GEMINI_API_KEY:=}
+  - provider_id: ${env.VERTEX_AI_PROJECT:+vertexai}
+    provider_type: remote::vertexai
+    config:
+      project: ${env.VERTEX_AI_PROJECT:=}
+      location: ${env.VERTEX_AI_LOCATION:=us-central1}
   - provider_id: groq
     provider_type: remote::groq
     config:

llama_stack/distributions/starter/starter.py

@@ -56,6 +56,7 @@ ENABLED_INFERENCE_PROVIDERS = [
     "fireworks",
     "together",
     "gemini",
+    "vertexai",
     "groq",
     "sambanova",
     "anthropic",
@@ -71,6 +72,7 @@ INFERENCE_PROVIDER_IDS = {
     "tgi": "${env.TGI_URL:+tgi}",
     "cerebras": "${env.CEREBRAS_API_KEY:+cerebras}",
     "nvidia": "${env.NVIDIA_API_KEY:+nvidia}",
+    "vertexai": "${env.VERTEX_AI_PROJECT:+vertexai}",
 }
 
 
@@ -246,6 +248,14 @@ def get_distribution_template() -> DistributionTemplate:
                 "",
                 "Gemini API Key",
             ),
+            "VERTEX_AI_PROJECT": (
+                "",
+                "Google Cloud Project ID for Vertex AI",
+            ),
+            "VERTEX_AI_LOCATION": (
+                "us-central1",
+                "Google Cloud Location for Vertex AI",
+            ),
             "SAMBANOVA_API_KEY": (
                 "",
                 "SambaNova API Key",