Do not store mtp_losses/mtp_acceptance in state params

[bvandermoon]

Description

• Do not store mtp_losses and mtp_acceptance as part of params. This was causing a checkpoint loading error because they are not stored as part of the checkpoints.
• Update self attribute names to match pre-NNX checkpoints

FIXES: b/483723849

Tests

Successfully ran the XPK workload in the linked bug with checkpoint loading.

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/layers/multi_token_prediction.py	92.85%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[parambole]

LGTM. Thank you for the quick fix.

[parambole]

Thank you for the fix!

[suexu1025]

Thanks for the fix!