W&B: Allow for passing experiment into the WandbLogger (and logging semantics)
Currently, the WandbLogger will automatically create a new internal experiment (run) whenever you create a new WandbLogger.
Issue
If I instantiate a wandb experiment outside of the logger, then I will have two experiments when I train my model since there is no way to set the internal experiment of the WandbLogger to my current external experiment.
Potential Solution
Allow for passing an experiment into the WandbLogger:
class WandbLogger(LightningLoggerBase):
    def __init__(self, name=None, save_dir=None, offline=False, id=None, anonymous=False,
                 version=None, project=None, tags=None, experiment=None):
              .
              .
              .
              self._experiment = experiment

Then I can do this:
experiment = wandb.init(.......)
wandb_logger = WandbLogger(experiment=experiment)

I made this change locally, however, I wasn't sure if this was something you also wanted to implement as well. It works for me.
Another small note
In the WandbLogger.log_metrics function, I would change:
self.experiment.history.add(metrics) --> self.experiment.log(metrics)