Using multiple losses/datasets on the same model grows the update dictionary

[ppoulin91]

In class Loss:

@property
def losses(self):
    """ Gets individual losses (one for each batch example). """
    if self._losses is None:
        model_output = self.model.get_output(self.dataset.symb_inputs)
        self._losses = self._compute_losses(model_output)

    return self._losses

Bug example:
# Get train losses
train_loss = DummyLoss(model, trainset)
train_losses = train_loss.losses

# Get valid losses
valid_loss = DummyLoss(model, validset)
valid_losses = valid_loss.losses

The call to train_loss.losses triggers model.get_output, using trainset.symb_inputs
The model updates dictionary contains elements specific to trainset.symb_inputs

The call to valid_loss.losses triggers again model.get_output, this time using validset.symb_inputs
The model will update the updates dictionary, which now contains elements specific to both trainset.symb_inputs and validset.symb_inputs

The easy fix would be to force models to replace the updates dictionary. However, it would be preferable to find a better way of managing the updates and losses