Add auto save ckpt at the end of training (#260)

Currently, checkpoints will not be saved until the training steps
satisfy the saving frequency. This PR adds an auto-save ckpt function at
the end of training.

verl/trainer/ppo/ray_trainer.py

@@ -964,4 +964,8 @@ def fit(self):
                         val_metrics = self._validate()
                         pprint(f'Final validation metrics: {val_metrics}')
                         logger.log(data=val_metrics, step=self.global_steps)
+                    if self.config.trainer.save_freq > 0 and \
+                            (self.global_steps - 1) % self.config.trainer.save_freq != 0:
+                        with _timer('save_checkpoint', timing_raw):
+                            self._save_checkpoint()
                     return