fix failing unit tests for no torch flavors (#2317)

darts/tests/models/forecasting/test_torch_forecasting_model.py

@@ -7,7 +7,6 @@
 import numpy as np
 import pandas as pd
 import pytest
-from pytorch_lightning.callbacks import Callback
 
 import darts.utils.timeseries_generation as tg
 from darts import TimeSeries
@@ -21,6 +20,7 @@
 
 try:
     import torch
+    from pytorch_lightning.callbacks import Callback
     from pytorch_lightning.loggers.logger import DummyLogger
     from pytorch_lightning.tuner.lr_finder import _LRFinder
     from torchmetrics import (