Confusion about Decoder labels for training seq-to-seq models
It can be both. 

If you input the desired label and predict the next desired label, it's called teacher forcing.

But using only this technique might hurt the performance at test time. So using the actual predicted output from the last time step is also a good idea.

It's possible to do both :
For each batch, with X% chance you use teacher forcing, otherwise you don't.