Solvers logging (#202)

* Modifying solvers to log every epoch correctly
* add `on_epoch` flag to logger
* fix bug in `pinn.py` `pts -> samples` in `_loss_phys`
* add `optimizer_zero_grad()` in garom generator training loop
* modify imports in `callbacks.py`
* fixing tests

---------

Co-authored-by: Dario Coscia <dariocoscia@dhcp-015.eduroam.sissa.it>

pina/callbacks/processing_callbacks.py

@@ -1,6 +1,6 @@
 '''PINA Callbacks Implementations'''
 
-from lightning.pytorch.callbacks import Callback
+from pytorch_lightning.callbacks import Callback
 import torch
 import copy