equation class, fix minor bugs, diff domain (#89)

* equation class
* difference domain
* dummy dataloader
* writer class
* refactoring and minor fix

pina/label_tensor.py

@@ -1,5 +1,7 @@
 """ Module for LabelTensor """
+from typing import Any
 import torch
+from torch import Tensor
 
 
 class LabelTensor(torch.Tensor):
@@ -79,7 +81,7 @@ class LabelTensor(torch.Tensor):
 
     @labels.setter
     def labels(self, labels):
-        if len(labels) != self.shape[1]:  # small check
+        if len(labels) != self.shape[self.ndim - 1]:  # small check
             raise ValueError(
                 'the tensor has not the same number of columns of '
                 'the passed labels.')
@@ -140,7 +142,7 @@ class LabelTensor(torch.Tensor):
             except ValueError:
                 raise ValueError(f'`{f}` not in the labels list')
 
-        new_data = self[:, indeces].float()
+        new_data = super(Tensor, self.T).__getitem__(indeces).float().T
         new_labels = [self.labels[idx] for idx in indeces]
 
         extracted_tensor = new_data.as_subclass(LabelTensor)
@@ -183,6 +185,19 @@ class LabelTensor(torch.Tensor):
         new_tensor.labels = new_labels
         return new_tensor
 
+    def __getitem__(self, index):
+        """
+        Return a copy of the selected tensor.
+        """
+        selected_lt = super(Tensor, self).__getitem__(index)
+        if hasattr(self, 'labels'):
+            selected_lt.labels = self.labels
+
+        return selected_lt
+
+    def __len__(self) -> int:
+        return super().__len__()
+        
     def __str__(self):
         if hasattr(self, 'labels'):
             s = f'labels({str(self.labels)})\n'