[BUG] Some Models return a straight-line

[ArchanGhosh]

Describe the bug I am trying to get some predictions and for that I have used around 3 model(AutoArima, FFT, Theta) and now I choose the one with the lowest MAPE value. But sometimes it is only giving a single value over a range of predictions, and the outcome is a straight-line.

Expected behavior if the Mape value is low hence it should produce atleast some pattern

System (please complete the following information):

• Python version: 3.7
• darts version 0.21.0