Add my new, shiny module.

llm_harness_mistral_arc.py

@@ -27,7 +27,7 @@ class llm_harness_mistral_arc(evaluate.Metric):
             citation=_CITATION,
             inputs_description=_KWARGS_DESCRIPTION,
             # This defines the format of each prediction and reference
-            features=None,
+            features={},
             # Homepage of the module for documentation
             homepage="http://module.homepage",
             # Additional links to the codebase or references