N- beats theta value

[shoaibakhtar1010]

Output the backcast and forecast from theta

backcast, forecast = theta[:, :self.input_size], theta[:, -self.horizon:]

How the theta range is decided?

[UmbertoFasci]

Perhaps you can get some answers from this code:
N-Beats Simple Example

You might also want to explore the associated paper:
N-Beats Paper