code: def load_dataframe(self, df_loader_name):
          
          logger.debug("loading dataframe: {}".format(df_loader_name))
          # Get the DataFrameLoader object corresponding to this name.
          df_loaders = [df_loader for df_loader in self.df_loaders if df_loader.name == df_loader_name]
  
          if len(df_loaders) == 0:
              raise ValueError("No DataFrameLoader with name %s" % df_loader_name)
          if len(df_loaders) > 1:
              raise ValueError("Multiple DataFrameLoaders with name %s" % df_loader_name)
  
          return df_loaders[0].load_dataframe()
docs: Instead of joining a DataFrameJoiner with the Cohort in `as_dataframe`, sometimes
          we may want to just directly load a particular DataFrame.