gemma3 packing fixes (#2449)

* make gemma3 work with packing

* multi-gpu e2e for ci

* update gemma3 model namespace to use mirror

* add gradient checkpointing to multigpu e2e ci

* update gemma3 examples for use_reentrant and fix ddp find unused params

* fix tests for gemma3

* fix import for test utils

* set correct train loss for gemma3 e2e

src/axolotl/monkeypatch/multipack.py

@@ -22,6 +22,7 @@ SUPPORTED_MULTIPACK_MODEL_TYPES = [
     "phi3",
     "gemma",
     "gemma2",
+    "gemma3",
     "gemma3_text",
     "cohere",
     "cohere2",