Adjust Trainings#async_create signature

to better align with `Trainings.create` and the way that the arguments
are being used.

Closes #408

Author: meatballhat

replicate/training.py

@@ -307,9 +307,9 @@ def create(  # type: ignore
 
     async def async_create(
         self,
-        model: Union[str, Tuple[str, str], "Model"],
-        version: Union[str, Version],
-        input: Dict[str, Any],
+        model: Optional[Union[str, Tuple[str, str], "Model"]] = None,
+        version: Optional[Union[str, Version]] = None,
+        input: Optional[Dict[str, Any]] = None,
         **params: Unpack["Trainings.CreateTrainingParams"],
     ) -> Training:
         """
@@ -326,7 +326,15 @@ async def async_create(
             The training object.
         """
 
-        url = _create_training_url_from_model_and_version(model, version)
+        url = None
+
+        if model and version:
+            url = _create_training_url_from_model_and_version(model, version)
+        elif model is None and isinstance(version, str):
+            url = _create_training_url_from_shorthand(version)
+
+        if not url:
+            raise ValueError("model and version or shorthand version must be specified")
 
         file_encoding_strategy = params.pop("file_encoding_strategy", None)
         if input is not None: