add download method to base class

lm_eval/base.py

@@ -54,6 +54,12 @@ class LM(abc.ABC):
 
 
 class Dataset(abc.ABC):
+
+    @abc.abstractmethod
+    def download(self):
+        """Downloads the task dataset if necessary"""
+        pass
+
     @abc.abstractmethod
     def has_training_docs(self):
         """Whether the task has a training set"""
@@ -121,4 +127,4 @@ class Dataset(abc.ABC):
             map(self.doc_to_text, self.fewshot_examples(k=num_fewshot))
         ) + "\n\n"
         example = self.doc_to_text(doc, include_target=False).strip()
-        return description + labeled_examples + example
+        return description + labeled_examples + example
\ No newline at end of file