Raise deprecation warning

c4.py

@@ -3,6 +3,7 @@
 
 import gzip
 import json
+import warnings
 
 import datasets
 
@@ -49,6 +50,10 @@ class C4(datasets.GeneratorBasedBuilder):
     BUILDER_CONFIGS = [datasets.BuilderConfig(name) for name in _VARIANTS]
 
     def _info(self):
+        warnings.warn(
+            "Dataset 'c4' is deprecated and will be deleted. Use 'allenai/c4' instead.",
+            FutureWarning,
+        )
         return datasets.DatasetInfo(
             description=_DESCRIPTION,
             features=datasets.Features(