docstr: Initialise online state (the stateful attributes updated by `score` and `predict`). This method relies on
  attributes defined by `_configure_ref_subset`, hence must be called afterwards.
docstr_tok: ["Initialise","online","state","(","the","stateful","attributes","updated","by","`","score","`","and","`","predict","`",")",".","This","method","relies","on","attributes","defined","by","`","_configure_ref_subset","`",",","hence","must","be","called","afterwards","."]
code: def _initialise_state(self) -> None:
      """
      Initialise online state (the stateful attributes updated by `score` and `predict`). This method relies on
      attributes defined by `_configure_ref_subset`, hence must be called afterwards.
      """
      super()._initialise_state()
      self.test_window = tf.gather(self.x_ref, self.init_test_inds)
      self.k_xy = self.kernel(
          tf.gather(self.x_ref, self.ref_inds), self.test_window
      )
  
file_path: alibi-detect/alibi_detect/cd/tensorflow/mmd_online.py
imports: from tqdm import tqdm
  import numpy
  import tensorflow
  from typing import Any, Callable, Optional, Union
  from alibi_detect.cd.base_online import BaseMultiDriftOnline
  from alibi_detect.utils.tensorflow.kernels import GaussianRBF
  from alibi_detect.utils.tensorflow import zero_diag, quantile, subset_matrix
  from alibi_detect.utils.frameworks import Framework
label_desc: Use image node_id 2 for calling the MMDDriftOnlineTF obj's underlying member method code with example usage: obj._initialise_state() without return types