Update Google-specific imports

PiperOrigin-RevId: 874404593

Author: bvandermoon

src/maxtext/trainers/pre_train/train.py

@@ -42,6 +42,7 @@
 from MaxText import sharding
 from MaxText.common_types import ShardMode
 from MaxText.globals import EPS
+# Placeholder: internal
 
 from MaxText.gradient_accumulation import gradient_accumulation_loss_and_grad
 from MaxText.vocabulary_tiling import vocab_tiling_linen_loss