Trainer.scale_batch_size requires model.batch_size instead of model.hparams.batch_size
🐛 Bug

Trainer.scale_batch_size only works if a model has the batch_size property and does not work with model.hparams.batch_size even though all documentation points to the reverse.
To Reproduce
All of my hyperparameters are available as model.hparams like suggested in the documentation: (hyperparameters, option 3.
This means that my batch_size is available as model.hparams.batch_size.
This should be fully compatible with the documented example code of Trainer.scale_batch_size() since that code also uses model.hparams.batch_size instead of model.batch_size.
However, when I put my model in Trainer.scale_batch_size, I get the following error:
pytorch_lightning.utilities.exceptions.MisconfigurationException: Field batch_size not found in `model.hparams`

Example code
class LitModel(pl.LightningModule):
    def __init__(self, hparams):
        super().__init__()
        self.hparams = args

model = LitModel(args)
trainer = Trainer()
trainer.scale_batch_size(model)

Expected behavior
Either Trainer.scale_batch_size should work with model.hparams or the error message, linked documentation examples and docstrings should all change (i.e. here, here and here).
(I would prefer the second option. I think that it should work with both model.batch_size and model.hparams.batch_size.)
Environment

pytorch-lightning         0.8.4