message: Add a logging entry to help diagnose the cause of "<var name> must be defined before the loop" errors coming from autograph.
diff: @@ -1019,6 +1019,7 @@ def _try_handling_undefineds(
  body()
  first_iter_vars = get_state()
  except (UnboundLocalError, TypeError, ValueError, KeyError):
  + ag_logging.log(1, 'Caught error while staging loop body', exc_info=True)
  # Fall back to the old functionality. It will likely result in an input
  # validation failure.
  first_iter_vars = None
  