Explain the configurations of the Transformer decoder please!

Firstly, thank you for the awesome project. I am a bit confused about why you configured the Transformer decoder like this:
```
  d_model: 128
  dim_feedforward: 256
  nhead: 4
  dropout: 0.3
  num_decoder_layers: 3
  max_output_len: 150
```
How can I configure these parameters correctly?
Thank you for reading, and I hope you will respond to me soon.
@kingyiusuen 