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/llama-3/sparse-finetuning.yaml

@@ -75,3 +75,5 @@ llmcompressor:
           ]
           start: 0
   save_compressed: true
+
+# save_first_step: true  # uncomment this to validate checkpoint saving works with your config