Group argument wandb (#1760)

* group argument wandb

* formatting fix

pytorch_lightning/loggers/wandb.py

@@ -39,6 +39,7 @@ class WandbLogger(LightningLoggerBase):
         log_model: Save checkpoints in wandb dir to upload on W&B servers.
         experiment: WandB experiment object
         entity: The team posting this run (default: your username or your default team)
+        group: A unique string shared by all runs in a given group
 
     Example:
         >>> from pytorch_lightning.loggers import WandbLogger
@@ -64,7 +65,8 @@ class WandbLogger(LightningLoggerBase):
                  tags: Optional[List[str]] = None,
                  log_model: bool = False,
                  experiment=None,
-                 entity=None):
+                 entity=None,
+                 group: Optional[str] = None):
         super().__init__()
         self._name = name
         self._save_dir = save_dir
@@ -76,6 +78,7 @@ class WandbLogger(LightningLoggerBase):
         self._offline = offline
         self._entity = entity
         self._log_model = log_model
+        self._group = group
 
     def __getstate__(self):
         state = self.__dict__.copy()
@@ -103,7 +106,8 @@ class WandbLogger(LightningLoggerBase):
                 os.environ['WANDB_MODE'] = 'dryrun'
             self._experiment = wandb.init(
                 name=self._name, dir=self._save_dir, project=self._project, anonymous=self._anonymous,
-                reinit=True, id=self._id, resume='allow', tags=self._tags, entity=self._entity)
+                reinit=True, id=self._id, resume='allow', tags=self._tags, entity=self._entity,
+                group=self._group)
             # save checkpoints in wandb dir to upload on W&B servers
             if self._log_model:
                 self.save_dir = self._experiment.dir