Issues with saving checkpoints
Hi! I'm trying to train tacotron 2 from scratch.

Running in current main branch with pytorch-lightning==1.5.0
```
PYTHONPATH="$(pwd)" python examples/tts/tacotron2.py \
train_dataset=train_data.json \
validation_datasets=validation_data.json \
+exp_manager.checkpoint_callback_params.save_top_k=1 \
+exp_manager.checkpoint_callback_params.monitor=loss
```

Results in correct training without checkpointing. Downgrading to pytorch-lightning==1.4.2 fixes this issue.