Fix type check.

Author: babacry

cirq-core/cirq/transformers/tag_transformers.py

@@ -26,7 +26,7 @@ def index_tags(
     circuit: 'cirq.AbstractCircuit',
     *,
     context: Optional['cirq.TransformerContext'] = None,
-    target_tags: set[Hashable],
+    target_tags: Optional[set[Hashable]] = None,
 ) -> 'cirq.Circuit':
     """Indexes tags in target_tags as tag_0, tag_1, ... per tag.