Base64Codec support single value

mlserver/codecs/base64.py

@@ -5,7 +5,7 @@
 from functools import partial
 
 from ..types import RequestInput, ResponseOutput
-from .utils import is_list_of
+from .utils import is_list_of, is_single_value
 from .base import InputCodec, register_input_codec
 from .pack import unpack, PackElement
 
@@ -66,8 +66,14 @@ def encode_output(
         )
 
     @classmethod
-    def decode_output(cls, response_output: ResponseOutput) -> List[bytes]:
+    def decode_output(
+        cls, response_output: ResponseOutput
+    ) -> Union[List[bytes], bytes]:
         packed = response_output.data.__root__
+        if is_single_value(response_output):
+            if len(packed) == 0:
+                raise ValueError("can't decode empty data as a single value")
+            return next(map(_decode_base64, unpack(packed)))
         return list(map(_decode_base64, unpack(packed)))
 
     @classmethod
@@ -84,7 +90,10 @@ def encode_input(
         )
 
     @classmethod
-    def decode_input(cls, request_input: RequestInput) -> List[bytes]:
+    def decode_input(cls, request_input: RequestInput) -> Union[List[bytes], bytes]:
         packed = request_input.data.__root__
-
+        if is_single_value(request_input):
+            if len(packed) == 0:
+                raise ValueError("can't decode empty data as a single value")
+            return next(map(_decode_base64, unpack(packed)))
         return list(map(_decode_base64, unpack(packed)))