Update tutorials (#463)

---------

Co-authored-by: Dario Coscia <93731561+dario-coscia@users.noreply.github.com>

tutorials/tutorial12/tutorial.py

@@ -38,16 +38,16 @@ except:
 if IN_COLAB:
   get_ipython().system('pip install "pina-mathlab"')
 
+import torch
+
 #useful imports
 from pina.problem import SpatialProblem, TimeDependentProblem
 from pina.equation import Equation, FixedValue
 from pina.domain import CartesianDomain
-import torch
-from pina.operators import grad, laplacian
+from pina.operator import grad, laplacian
 from pina import Condition
 
 
-
 # In[2]: