add POD-NN model

model/pod_nn.py

@@ -0,0 +1,22 @@
+import torch
+from pina.model.block import PODBlock
+from pina.model import FeedForward
+
+class PODNN(torch.nn.Module):
+    def __init__(self, pod_rank, layers, func=torch.nn.Softplus):
+        super().__init__()
+        self.pod = PODBlock(pod_rank, scale_coefficients=False)
+        self.nn = FeedForward(
+            input_dimensions=3,
+            output_dimensions=pod_rank,
+            layers=layers,
+            func=func,
+        )
+        
+
+    def forward(self, p):
+        coefficients = self.nn(p)
+        return self.pod.expand(coefficients)
+
+    def fit_pod(self, x):
+        self.pod.fit(x)