class_docstr: An abstract class for providing the basic structure of Innovation State
  Space Model (ISSM).
  
  The structure of ISSM is given by
  
      * dimension of the latent state
      * transition and innovation coefficients of the transition model
      * emission coefficient of the observation model
code: def time_features(self) -> List[TimeFeature]:
      raise NotImplementedError
  
imports: from typing import List, Sequence, Tuple
  from pandas.tseries.frequencies import to_offset
  from gluonts.core.component import validated
  from gluonts.mx import Tensor
  from gluonts.mx.distribution.distribution import getF
  from gluonts.mx.util import _broadcast_param
  from gluonts.time_feature import Constant, day_of_week_index, hour_of_day_index, minute_of_hour_index, month_of_year_index, TimeFeature, week_of_year_index, norm_freq_str
label_desc: Use image node_id 4 for calling the ISSM obj's underlying member method code with example usage: obj.time_features() without return types