auto_scale_batch_size fails with datamodule in pl==1.2.*, succeeds in pl==1.1.8
🐛 Bug
Running trainer = pl.Trainer(auto_scale_batch_size=True); trainer.tune(model, datamodule=dm) succeeds in pl==1.1.8, but fails in pl==1.2.* (tested both 1.2.0 and 1.2.1) with error:
Traceback (most recent call last):
  File "train.py", line 42, in <module>
    trainer.tune(model, datamodule=dm)
  File "/home/ubuntu/.local/share/virtualenvs/__project__/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1062, in tune
    self.tuner.tune(model, train_dataloader, val_dataloaders, datamodule)
  File "/home/ubuntu/.local/share/virtualenvs/__project__/lib/python3.8/site-packages/pytorch_lightning/tuner/tuning.py", line 46, in tune
    self.scale_batch_size(
  File "/home/ubuntu/.local/share/virtualenvs/__project__/lib/python3.8/site-packages/pytorch_lightning/tuner/tuning.py", line 104, in scale_batch_size
    return scale_batch_size(
  File "/home/ubuntu/.local/share/virtualenvs/__project__/lib/python3.8/site-packages/pytorch_lightning/tuner/batch_size_scaling.py", line 79, in scale_batch_size
    raise MisconfigurationException(f'Field {batch_arg_name} not found in both `model` and `model.hparams`')
pytorch_lightning.utilities.exceptions.MisconfigurationException: Field batch_size not found in both `model` and `model.hparams` 

Please reproduce using the BoringModel
https://colab.research.google.com/drive/1vgPLCwLg7uACtb3fxVp-t-__NtZ3onsD?usp=sharing
Expected behavior
Prior to pl==1.2.0, it successfully detected and tuned the dm.batch_size property.
Environment

CUDA:

GPU:

Tesla T4


available:         True
version:           10.1


Packages:

numpy:             1.19.5
pyTorch_debug:     False
pyTorch_version:   1.7.1+cu101
pytorch-lightning: 1.2.1
tqdm:              4.41.1


System:

OS:                Linux
architecture:

64bit



processor:         x86_64
python:            3.7.10
version:           #1 SMP Thu Jul 23 08:00:38 PDT 2020