Resume training fails with BackboneFinetuning callback.
🐛 Bug
Unable to resuming training when using BackboneFinetuning callback.
AttributeError: 'BackboneFinetuning' object has no attribute 'previous_backbone_lr'
To Reproduce
https://colab.research.google.com/drive/1RLKD3ISkGmAcy308GWUF57UTAoavdVcr?usp=sharing
Expected behavior
BackboneFinetuning state should be saved in checkpoint, and be resumed.
Environment

CUDA:

GPU:

Tesla T4


available:         True
version:           10.1


Packages:

numpy:             1.19.5
pyTorch_debug:     False
pyTorch_version:   1.8.0+cu101
pytorch-lightning: 1.2.3
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