[Doc]: Make extraInit containers fully configurable in helm chart (#27497)

Signed-off-by: Fang Han <fhan0520@gmail.com>

examples/online_serving/chart-helm/values.yaml

@@ -75,10 +75,65 @@ maxUnavailablePodDisruptionBudget: ""
 
 # -- Additional configuration for the init container
 extraInit:
-   # -- Path of the model on the s3 which hosts model weights and config files
+  # -- Model download functionality (optional)
+  modelDownload:
+    # -- Enable model download job and wait container
+    enabled: true
+    # -- Image configuration for model download operations
+    image:
+      # -- Image repository
+      repository: "amazon/aws-cli"
+      # -- Image tag
+      tag: "2.6.4"
+      # -- Image pull policy
+      pullPolicy: "IfNotPresent"
+    # -- Wait container configuration (init container that waits for model to be ready)
+    waitContainer:
+      # -- Command to execute
+      command: ["/bin/bash"]
+      # -- Arguments for the wait container
+      args:
+        - "-eucx"
+        - "while aws --endpoint-url $S3_ENDPOINT_URL s3 sync --dryrun s3://$S3_BUCKET_NAME/$S3_PATH /data | grep -q download; do sleep 10; done"
+      # -- Environment variables (optional, overrides S3 defaults entirely if specified)
+      # env:
+      #   - name: HUGGING_FACE_HUB_TOKEN
+      #     value: "your-token"
+      #   - name: MODEL_ID
+      #     value: "meta-llama/Llama-2-7b"
+    # -- Download job configuration (job that actually downloads the model)
+    downloadJob:
+      # -- Command to execute
+      command: ["/bin/bash"]
+      # -- Arguments for the download job
+      args:
+        - "-eucx"
+        - "aws --endpoint-url $S3_ENDPOINT_URL s3 sync s3://$S3_BUCKET_NAME/$S3_PATH /data"
+      # -- Environment variables (optional, overrides S3 defaults entirely if specified)
+      # env:
+      #   - name: HUGGING_FACE_HUB_TOKEN
+      #     value: "your-token"
+      #   - name: MODEL_ID
+      #     value: "meta-llama/Llama-2-7b"
+
+  # -- Custom init containers (appended after wait-download-model if modelDownload is enabled)
+  initContainers: []
+  # Example for llm-d sidecar:
+  # initContainers:
+  #   - name: llm-d-routing-proxy
+  #     image: ghcr.io/llm-d/llm-d-routing-sidecar:v0.2.0
+  #     imagePullPolicy: IfNotPresent
+  #     ports:
+  #       - containerPort: 8080
+  #         name: proxy
+  #     securityContext:
+  #       runAsUser: 1000
+
+  # -- Path of the model on the s3 which hosts model weights and config files
   s3modelpath: "relative_s3_model_path/opt-125m"
-   # -- Storage size of the s3
+  # -- Storage size for the PVC
   pvcStorage: "1Gi"
+  # -- Disable AWS EC2 metadata service
   awsEc2MetadataDisabled: true
 
 # -- Additional containers configuration