Fix checkpoint saving using wrong interval config

[Mr-Neutr0n]

Summary

The checkpoint saving condition in train.py (line 242) has a bug where it checks config.checkpoint_every to determine whether checkpointing is enabled, but then incorrectly uses config.eval_every for the modulo operation:

# Before (bug):
if ((config.checkpoint_every and step % config.eval_every == 0) or
    step == total_steps):

# After (fix):
if ((config.checkpoint_every and step % config.checkpoint_every == 0) or
    step == total_steps):

This causes checkpoints to be saved at evaluation intervals instead of the configured checkpoint intervals, making the checkpoint_every config value effectively ignored for controlling save frequency.

Test plan

• Verified the fix by inspecting the changed line
• The fix is a single-token change from config.eval_every to config.checkpoint_every in the modulo check, matching the guard condition on the same line

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