Fix undefined trackers property (#224)

This bug was introduced by #209, which changed the `trackers` config property to `tracker` but didn't update this use case.

Author: alan-cooney

trlx/trainer/accelerate_base_trainer.py

@@ -418,7 +418,7 @@ def evaluate(self):  # noqa: C901
                 rich_table.add_row(*[str(significant(x)) for x in rows[ix]])
 
             if not ray.is_initialized():
-                if "wandb" in self.config.train.trackers:
+                if self.config.train.tracker == "wandb":
                     import wandb
 
                     stats["samples"] = wandb.Table(columns, rows)