* Adding a test for all PINN solvers to assert that the metrics are correctly log

* Adding test for Metric Tracker
* Modify Metric Tracker to correctly log metrics

tests/test_callbacks/test_metric_tracker.py

@@ -0,0 +1,87 @@
+import torch
+import pytest
+
+from pina.problem import SpatialProblem
+from pina.operators import laplacian
+from pina.geometry import CartesianDomain
+from pina import Condition, LabelTensor
+from pina.solvers import PINN
+from pina.trainer import Trainer
+from pina.model import FeedForward
+from pina.equation.equation import Equation
+from pina.equation.equation_factory import FixedValue
+from pina.callbacks import MetricTracker
+
+
+def laplace_equation(input_, output_):
+    force_term = (torch.sin(input_.extract(['x']) * torch.pi) *
+                  torch.sin(input_.extract(['y']) * torch.pi))
+    delta_u = laplacian(output_.extract(['u']), input_)
+    return delta_u - force_term
+
+
+my_laplace = Equation(laplace_equation)
+in_ = LabelTensor(torch.tensor([[0., 1.]]), ['x', 'y'])
+out_ = LabelTensor(torch.tensor([[0.]]), ['u'])
+
+
+class Poisson(SpatialProblem):
+    output_variables = ['u']
+    spatial_domain = CartesianDomain({'x': [0, 1], 'y': [0, 1]})
+
+    conditions = {
+        'gamma1': Condition(
+            location=CartesianDomain({'x': [0, 1], 'y':  1}),
+            equation=FixedValue(0.0)),
+        'gamma2': Condition(
+            location=CartesianDomain({'x': [0, 1], 'y': 0}),
+            equation=FixedValue(0.0)),
+        'gamma3': Condition(
+            location=CartesianDomain({'x':  1, 'y': [0, 1]}),
+            equation=FixedValue(0.0)),
+        'gamma4': Condition(
+            location=CartesianDomain({'x': 0, 'y': [0, 1]}),
+            equation=FixedValue(0.0)),
+        'D': Condition(
+            input_points=LabelTensor(torch.rand(size=(100, 2)), ['x', 'y']),
+            equation=my_laplace),
+        'data': Condition(
+            input_points=in_,
+            output_points=out_)
+    }
+
+
+# make the problem
+poisson_problem = Poisson()
+boundaries = ['gamma1', 'gamma2', 'gamma3', 'gamma4']
+n = 10
+poisson_problem.discretise_domain(n, 'grid', locations=boundaries)
+model = FeedForward(len(poisson_problem.input_variables),
+                    len(poisson_problem.output_variables))
+
+# make the solver
+solver = PINN(problem=poisson_problem, model=model)
+
+
+def test_metric_tracker_constructor():
+    MetricTracker()
+
+def test_metric_tracker_routine():
+    # make the trainer
+    trainer = Trainer(solver=solver,
+                      callbacks=[
+                          MetricTracker()
+                      ],
+                      accelerator='cpu',
+                      max_epochs=5)
+    trainer.train()
+    # get the tracked metrics
+    metrics = trainer.callbacks[0].metrics
+    # assert the logged metrics are correct
+    logged_metrics = sorted(list(metrics.keys()))
+    total_metrics = sorted(
+        list([key + '_loss' for key in poisson_problem.conditions.keys()])
+        + ['mean_loss'])
+    assert logged_metrics == total_metrics
+
+