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

examples/mistral/mixtral_22.yml

@@ -51,3 +51,5 @@ special_tokens:
   eos_token: "<|im_end|>"
 tokens:
   - "<|im_start|>"
+
+# save_first_step: true  # uncomment this to validate checkpoint saving works with your config