Cache Image.format for better throughput (#8842)

* cache adapter methods for better throuput

* cache image type

* revert base adapter change

Author: TomeHirata

dspy/adapters/types/image.py

@@ -3,6 +3,7 @@
 import mimetypes
 import os
 import warnings
+from functools import lru_cache
 from typing import Any, Union
 from urllib.parse import urlparse
 
@@ -65,6 +66,7 @@ def __init__(self, url: Any = None, *, download: bool = False, **data):
         # Delegate the rest of initialization to pydantic's BaseModel.
         super().__init__(**data)
 
+    @lru_cache(maxsize=32)
     def format(self) -> list[dict[str, Any]] | str:
         try:
             image_url = encode_image(self.url)