Fourier Block and minor fixes

* Adding fourier block 1d/2d/3d
* Adding docs to SpectralConvBlock1D/2D/3D and to FourierBlock1D/2D/3D
* Adding tests for fourier block

tests/test_layers/test_fourier.py

@@ -0,0 +1,43 @@
+from pina.model.layers import FourierBlock1D, FourierBlock2D, FourierBlock3D
+import torch
+
+input_numb_fields = 3
+output_numb_fields = 4
+batch = 5
+
+def test_constructor_1d():
+    FourierBlock1D(input_numb_fields=input_numb_fields,
+                        output_numb_fields=output_numb_fields,
+                        n_modes=5)
+    
+def test_forward_1d():
+    sconv = FourierBlock1D(input_numb_fields=input_numb_fields,
+                                output_numb_fields=output_numb_fields,
+                                n_modes=4)
+    x = torch.rand(batch, input_numb_fields, 10)
+    sconv(x)
+
+
+def test_constructor_2d():
+    FourierBlock2D(input_numb_fields=input_numb_fields,
+                        output_numb_fields=output_numb_fields,
+                        n_modes=[5, 4])
+    
+def test_forward_2d():
+    sconv = FourierBlock2D(input_numb_fields=input_numb_fields,
+                                output_numb_fields=output_numb_fields,
+                                n_modes=[5, 4])
+    x = torch.rand(batch, input_numb_fields, 10, 10)
+    sconv(x)
+
+def test_constructor_3d():
+    FourierBlock3D(input_numb_fields=input_numb_fields,
+                        output_numb_fields=output_numb_fields,
+                        n_modes=[5, 4, 4])
+    
+def test_forward_3d():
+    sconv = FourierBlock3D(input_numb_fields=input_numb_fields,
+                                output_numb_fields=output_numb_fields,
+                                n_modes=[5, 4, 4])
+    x = torch.rand(batch, input_numb_fields, 10, 10, 10)
+    sconv(x)