Question regarding the transformer-architecture

I am trying to implement transformers in pytorch from scratch. If we feed into the decoder block what the transformer had previously generated. In my understanding the output of the decoder block should be of dimension(acc to the tutorial referenced below)

(batch_size, Ty, trg_vocab_size) 

The Ty is the len of inp to the decoder. Do we avg it? bc we want it to only generate one word at a time, right? Why is the output of the decoder(transformer block) dependent …

architecture deeplearning transformer