checkpoint model on first step callback (#2906)

* checkpoint model on first step callback

* remove debug

* add test cases; update existing tests not to save on first step

* move test out of solo

* delete

* default to False

* typo

tests/e2e/test_process_reward_model_smollm2.py

@@ -49,6 +49,7 @@ class TestProcessRewardSmolLM2(unittest.TestCase):
                 "use_tensorboard": True,
                 "special_tokens": {"pad_token": "<|endoftext|>"},
                 "seed": 42,
+                "save_first_step": False,
             }
         )
         cfg = validate_config(cfg)