Trainer tuning fails when used with a LightningDataModule

The `lr_find` call must take in `train_dataloader`, `val_dataloaders` and `datamodule` in order to support dataloaders separately.

https://github.com/PyTorchLightning/pytorch-lightning/blob/3e3175d0748ef461bf13b2258c8acdbe3a7bc8a5/pytorch_lightning/tuner/tuning.py#L64