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_mamba.py

@@ -51,6 +51,7 @@ class TestMamba(unittest.TestCase):
                 "save_steps": 10,
                 "eval_steps": None,
                 "save_safetensors": False,
+                "save_first_step": False,
             }
         )