fix(helm): Enable Backend Deployment to mount Gateway config.yaml (#29605)

* change deployment configs to include a litellm.cache for litellm-backend pod mirroring litellm-gateway pod

* omit backend annotations block when config and podAnnotations are both empty

* reuse gateway config/configmap for backend instead of separate backend config

---------

Co-authored-by: shin-berri <shin-laptop@berri.ai>
Co-authored-by: yuneng-jiang <yuneng@berri.ai>
Co-authored-by: Tin Chi Lo <tin@Tins-MBP.localdomain>
Co-authored-by: Tin Chi Lo <tin@Tins-MacBook-Pro.local>

helm/litellm/templates/backend/deployment.yaml

@@ -12,10 +12,15 @@ spec:
       {{- include "litellm.backend.selectorLabels" . | nindent 6 }}
   template:
     metadata:
-      {{- with .Values.backend.podAnnotations }}
+      {{- if or .Values.gateway.config.create .Values.backend.podAnnotations }}
       annotations:
+        {{- if .Values.gateway.config.create }}
+        checksum/config: {{ include (print $.Template.BasePath "/gateway/configmap.yaml") . | sha256sum }}
+        {{- end }}
+        {{- with .Values.backend.podAnnotations }}
         {{- toYaml . | nindent 8 }}
         {{- end }}
+      {{- end }}
       labels:
         {{- include "litellm.backend.selectorLabels" . | nindent 8 }}
     spec:
@@ -35,7 +40,17 @@ spec:
               protocol: TCP
           env:
             {{- include "litellm.serverEnv" (dict "root" $ "component" .Values.backend) | nindent 12 }}
+            {{- if .Values.gateway.config.create }}
+            - name: CONFIG_FILE_PATH
+              value: /app/config/config.yaml
+            {{- end }}
           {{- include "litellm.envFrom" .Values.backend | nindent 10 }}
+          {{- if .Values.gateway.config.create }}
+          volumeMounts:
+            - name: gateway-config
+              mountPath: /app/config/config.yaml
+              subPath: config.yaml
+          {{- end }}
           {{- with .Values.backend.livenessProbe }}
           livenessProbe:
             {{- toYaml . | nindent 12 }}
@@ -46,6 +61,12 @@ spec:
           {{- end }}
           resources:
             {{- toYaml .Values.backend.resources | nindent 12 }}
+      {{- if .Values.gateway.config.create }}
+      volumes:
+        - name: gateway-config
+          configMap:
+            name: {{ include "litellm.gateway.fullname" . }}-config
+      {{- end }}
       {{- with .Values.backend.nodeSelector }}
       nodeSelector:
         {{- toYaml . | nindent 8 }}