Implementation of DataLoader and DataModule (#383)

Refactoring for 0.2
* Data module, data loader and dataset
* Refactor LabelTensor
* Refactor solvers

Co-authored-by: dario-coscia <dariocos99@gmail.com>

pina/model/layers/lowrank_layer.py

@@ -119,6 +119,7 @@ class LowRankBlock(torch.nn.Module):
         :rtype: torch.Tensor
         """
         # extract basis
+        coords = coords.as_subclass(torch.Tensor)
         basis = self._basis(coords)
         # reshape [B, N, D, 2*rank]
         shape = list(basis.shape[:-1]) + [-1, 2 * self.rank]