Update csv_logs.py

Author: justusschock

pytorch_lightning/loggers/csv_logs.py

@@ -95,6 +95,8 @@ def save(self) -> None:
         metrics_keys = list(last_m.keys())
 
         with open(self.metrics_file_path, "w", newline="") as f:
+            # Don't assign the writer to self. 
+            # Keeps an open reference and prevents pickling otherwise
             writer = csv.DictWriter(f, fieldnames=metrics_keys)
             writer.writeheader()
             writer.writerows(self.metrics)