default_root_path vs default_root_dir
I noticed that the argument to Trainer is default_root_dir, but the docs sometimes show default_root_path. I did a grep on the codebase for default_root_path, and here are the results:
./notebooks/05-trainer-flags-overview.ipynb:2155:    "trainer = pl.Trainer(default_root_path=os.getcwd())\n",
./pytorch_lightning/trainer/properties.py:51:    default_root_path: str
./docs/source/trainer.rst:618:    trainer = Trainer(default_root_path=os.getcwd())

I'm fairly convinced it's simply a documentation issue, but it requires some code changes (the 2nd line), and I'm not sure whether that will affect anything.