refactor(benchmark): delegate model loading to tasks and support CIFAR-100

- Extract model loading logic from benchmark CLI into task-owned prepare_benchmark
- Add RetrievalEncoder class wrapping DINO with optional hash compression
- Add accelerate dependency for device management  
- Switch dataset from CIFAR-10 to CIFAR-100 with fine_label column

mini-nav/configs/config.yaml

@@ -24,9 +24,9 @@ dataset:
 benchmark:
     dataset:
         source_type: "huggingface"
-        path: "uoft-cs/cifar10"
+        path: "uoft-cs/cifar100"
         img_column: "img"
-        label_column: "label"
+        label_column: "fine_label"
     task:
         name: "recall_at_k"
         type: "retrieval"