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(...)

Tests!

Fully tested!

Includes:

• Code coverage (99%)
• Full tests that run multiple models in different configs
• Full tests that test specific functionality in trainer.