feat: Introduce weighted and rrf reranker implementations

Signed-off-by: Varsha Prasad Narsing <varshaprasad96@gmail.com>

docs/_static/llama-stack-spec.yaml

@@ -9758,6 +9758,11 @@ components:
           description: >-
             Search mode for retrieval—either "vector", "keyword", or "hybrid". Default
             "vector".
+        ranker:
+          $ref: '#/components/schemas/Ranker'
+          description: >-
+            Configuration for the ranker to use in hybrid search. Defaults to RRF
+            ranker.
       additionalProperties: false
       required:
         - query_generator_config
@@ -9776,6 +9781,58 @@ components:
         mapping:
           default: '#/components/schemas/DefaultRAGQueryGeneratorConfig'
           llm: '#/components/schemas/LLMRAGQueryGeneratorConfig'
+    RRFRanker:
+      type: object
+      properties:
+        type:
+          type: string
+          const: rrf
+          default: rrf
+          description: The type of ranker, always "rrf"
+        impact_factor:
+          type: number
+          default: 60.0
+          description: >-
+            The impact factor for RRF scoring. Higher values give more weight to higher-ranked
+            results. Must be greater than 0. Default of 60 is from the original RRF
+            paper (Cormack et al., 2009).
+      additionalProperties: false
+      required:
+        - type
+        - impact_factor
+      title: RRFRanker
+      description: >-
+        Reciprocal Rank Fusion (RRF) ranker configuration.
+    Ranker:
+      oneOf:
+        - $ref: '#/components/schemas/RRFRanker'
+        - $ref: '#/components/schemas/WeightedRanker'
+      discriminator:
+        propertyName: type
+        mapping:
+          rrf: '#/components/schemas/RRFRanker'
+          weighted: '#/components/schemas/WeightedRanker'
+    WeightedRanker:
+      type: object
+      properties:
+        type:
+          type: string
+          const: weighted
+          default: weighted
+          description: The type of ranker, always "weighted"
+        alpha:
+          type: number
+          default: 0.5
+          description: >-
+            Weight factor between 0 and 1. 0 means only use keyword scores, 1 means
+            only use vector scores, values in between blend both scores.
+      additionalProperties: false
+      required:
+        - type
+        - alpha
+      title: WeightedRanker
+      description: >-
+        Weighted ranker configuration that combines vector and keyword scores.
     QueryRequest:
       type: object
       properties: