Simple data loader

Simplified data loader.

Added a decorator to do lazy loading internally:

Old:

@property
def tng_dataloader(self):
      if self._tng_dataloader is None:
               self._tng_dataloader = DataLoader(...)
      return self.tng_dataloder

Now:

@ptl.data_loader
def tng_dataloader(self):
      return DataLoader(...)