make codec optional

Author: Dan-Flores

src/torchcodec/encoders/_video_encoder.py

@@ -36,7 +36,7 @@ def to_file(
         self,
         dest: Union[str, Path],
         *,
-        codec: str = None,
+        codec: Optional[str] = None,
         pixel_format: Optional[str] = None,
         crf: Optional[int] = None,
         preset: Optional[Union[str, int]] = None,
@@ -75,7 +75,7 @@ def to_tensor(
         self,
         format: str,
         *,
-        codec: str = None,
+        codec: Optional[str] = None,
         pixel_format: Optional[str] = None,
         crf: Optional[int] = None,
         preset: Optional[Union[str, int]] = None,
@@ -118,7 +118,7 @@ def to_file_like(
         file_like,
         format: str,
         *,
-        codec: str = None,
+        codec: Optional[str] = None,
         pixel_format: Optional[str] = None,
         crf: Optional[int] = None,
         preset: Optional[Union[str, int]] = None,