run_stokes example uses undefined attribute history

[Pratere]

Describe the bug
The example run_stokes.py attempts to save out history on ln 43:

for i, losses in enumerate(pinn.history):

The PINN class has no attribute history, this line results in an AttributeError.

To Reproduce
python examples/run_stokes.py -s 1

Expected behavior
A file to be saved with loss history for run.

Output

Traceback (most recent call last):
  File "/Users/eliprater/Documents/Projects/PINA/examples/run_stokes.py", line 43, in <module>
    for i, losses in enumerate(pinn.history):
AttributeError: 'PINN' object has no attribute 'history'

Additional context
Was it was meant for this line to reference the PINN attribute history_loss?

In that case the line could be changed to:

for i, losses in pinn.history_loss.items():

[Pratere]

Commit 936f5e1 changed the attribute history, which was a list, to history_loss, which is a dict.