Which input to use when generating a new sequence
As you have seen, normally you need a "special token" to be given to the decoder as the first element in its input to start the autoregressive generation.

However, given that your output are real (floating point) numbers, it is a bit trickier, as you are not dealing with a discrete token vocabulary where you could simply reserve a token for that.

I would suggest using a specific value, like $0.0$. Your model should be able to figure out the no influence of the $0.0$ in the first position.

Another option would be to learn the value to be used as first token. You would have an extra trainable parameter that you use as value for the first position.