Beam Search 7.Implement Beam Search on Python side

Implement Beam Search in graphs looks a bit hard for me to debug. Here I explore if it's easier to implement on python land.

things to figure out

• Can we easily access to decoder input?
  • Yes, step method has decoder_inputs, we can also access self.decoder_inputs
• What is encoder_input in the implementation
  • It's tweet token_ids.
• What is decoder_input in the implementation
  • It's reply token_ids, but isn't it decoder_output??