class_docstr: Tuner callback base class.
  
  To build your own callbacks, inherit `mz.auto.tuner.callbacks.Callback`
  and overrides corresponding methods.
  
  A run proceeds in the following way:
  
  - run start (callback)
  - build model
  - build end (callback)
  - fit and evaluate model
  - collect result
  - run end (callback)
  
  This process is repeated for `num_runs` times in a tuner.
docstr: Callback on run start stage.
  
  :param tuner: Tuner.
  :param sample: Sampled hyper space. Changes to this dictionary affects
      the model building process of the tuner.
docstr_tok: ["Callback","on","run","start","stage",".",":","param","tuner",":","Tuner",".",":","param","sample",":","Sampled","hyper","space",".","Changes","to","this","dictionary","affects","the","model","building","process","of","the","tuner","."]
code: def on_run_start(self, tuner: "matchzoo.auto.Tuner", sample: dict):
      """
      Callback on run start stage.
  
      :param tuner: Tuner.
      :param sample: Sampled hyper space. Changes to this dictionary affects
          the model building process of the tuner.
      """
  
imports: import matchzoo
  from matchzoo.engine.base_model import BaseModel
label_desc: Use image node_id 1 for calling the Callback obj's underlying member method code with example usage: obj.on_run_start(tuner, sample) without return types