WandbLogger `save_dir` should be set after creation

[carmocca]

🐛 Bug

Save dir is not defined on instantiation as the docs suggest: https://github.com/PyTorchLightning/pytorch-lightning/blob/16b958095849e5c03805918d66bf4a7e1d32da44/pytorch_lightning/loggers/wandb.py#L221 https://github.com/PyTorchLightning/pytorch-lightning/blob/16b958095849e5c03805918d66bf4a7e1d32da44/pytorch_lightning/loggers/wandb.py#L305

To Reproduce

from pytorch_lightning.loggers import WandbLogger

logger = WandbLogger(project="foo")
assert logger.save_dir is not None

Expected behavior

We want this because the LightningCLI tries to access the log_dir https://github.com/PyTorchLightning/pytorch-lightning/blob/16b958095849e5c03805918d66bf4a7e1d32da44/pytorch_lightning/utilities/cli.py#L430-L432 with https://github.com/PyTorchLightning/pytorch-lightning/blob/16b958095849e5c03805918d66bf4a7e1d32da44/pytorch_lightning/trainer/trainer.py#L2186 but it's not available on creation

Additional context

This can also be reproduced with

from pytorch_lightning.utilities.cli import LightningCLI
from tests.helpers import BoringModel

cli = LightningCLI(BoringModel, run=False, trainer_defaults={"max_epochs": 1})
cli.trainer.fit(cli.model)
print(cli.trainer.logger)

$ python bug.py --trainer.logger=WandbLogger
...
Traceback (most recent call last):
  File "/home/carmocca/git/pytorch-lightning/kk.py", line 5, in <module>
    cli.trainer.fit(cli.model)
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 724, in fit
    self._call_and_handle_interrupt(
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 677, in _call_and_handle_interrupt
    return trainer_fn(*args, **kwargs)
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 765, in _fit_impl
    results = self._run(model, ckpt_path=self.ckpt_path)
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 1128, in _run
    self._call_setup_hook()  # allow user to setup lightning_module in accelerator environment
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 1447, in _call_setup_hook
    self._call_callback_hooks("setup", stage=fn)
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/trainer/trainer.py", line 1590, in _call_callback_hooks
    fn(self, self.lightning_module, *args, **kwargs)
  File "/home/carmocca/git/pytorch-lightning/pytorch_lightning/utilities/cli.py", line 432, in setup
    assert log_dir is not None

cc @awaelchli @morganmcg1 @AyushExel @borisdayma @scottire @manangoel99

[manangoel99]

@carmocca thanks for pointing this out. I think we'll change how save_dir behaves in the next release as discussed in #12604 . The default value being None is brining up a bunch of issues in other places as well so we can keep it the same as the project since that is a required argument.
However, to make sure no existing workflows break meanwhile a deprecation warning will be raised that the behaviour will change while keeping the behaviour of save_dir the same for now. @borisdayma has mentioned here why defaulting it to wandb is not a good idea.

[Yingyan-Xu]

This error shows up again on the latest master (I checked just now.) if I use WandbLogger with save_dir as init_args specified in the config yaml file. Somehow the save_dir is overwritten here by kwargs['dir'] which is by default None. Changing the key from save_dir to dir in the config yaml fixes the problem but it doesn't work with older version and in your API documentation is still says save_dir.