docs: def get_sample_size(self, key=None):
          """ Returns the number of samples in the input data
              @ In, key, an optional 2-tuple specifying a min-max id pair
              used for determining which partition size should be
              returned. If not specified then the size of the entire data
              set will be returned.
              @ Out, an integer specifying the number of samples.
          """
          if key is None:
              return len(self.Y)
          else:
              return len(self.get_partitions(self.persistence)[key])