Update import from lightning.pytorch (#409)

* update import
* Remove unnecessary import return type

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Co-authored-by: Filippo Olivo <filippo@filippoolivo.com>

pina/callbacks/optimizer_callbacks.py

@@ -1,6 +1,6 @@
 """PINA Callbacks Implementations"""
 
-from pytorch_lightning.callbacks import Callback
+from lightning.pytorch.callbacks import Callback
 import torch
 from ..utils import check_consistency