Low Rank Neural Operator (#270)

* add the Low Rank Neural Operator as Model
* add the Low Rank Layer as Layer
* adding tests
* adding doc 

---------

Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>
Co-authored-by: Nicola Demo <demo.nicola@gmail.com>

docs/source/_rst/_code.rst

@@ -57,6 +57,7 @@ Models
     FourierIntegralKernel <models/fourier_kernel.rst>
     FNO <models/fno.rst>
     AveragingNeuralOperator <models/avno.rst>
+    LowRankNeuralOperator <models/lno.rst>
 
 Layers
 -------------
@@ -69,6 +70,7 @@ Layers
     Spectral convolution <layers/spectral.rst>
     Fourier layers <layers/fourier.rst>
     Averaging layer <layers/avno_layer.rst>
+    Low Rank layer <layers/lowrank_layer.rst>
     Continuous convolution <layers/convolution.rst>
     Proper Orthogonal Decomposition <layers/pod.rst>
     Periodic Boundary Condition embeddings <layers/embedding.rst>

docs/source/_rst/layers/lowrank_layer.rst

@@ -0,0 +1,8 @@
+Low Rank layer
+====================
+.. currentmodule:: pina.model.layers.lowrank_layer
+
+.. autoclass:: LowRankBlock
+    :members:
+    :show-inheritance:
+    :noindex:

docs/source/_rst/models/lno.rst

@@ -0,0 +1,7 @@
+Low Rank Neural Operator
+==============================
+.. currentmodule:: pina.model.lno
+
+.. autoclass:: LowRankNeuralOperator
+   :members:
+   :show-inheritance: