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/multigpu/test_gemma3.py

@@ -71,6 +71,7 @@ class TestMultiGPUGemma3:
                 "flash_attention": True,
                 "use_tensorboard": True,
                 "bf16": True,
+                "save_first_step": False,
             }
         )