Fix MTC NNX checkpoint restore

[SujeethJinesh]

Description

Since the recent MaxText move to NNX, restoration from emergency checkpointers (like Multi-Tiered Checkpointing) don't work in the intended way. This fixes it for the MTC path.

Without this fix, restoration gets: ValueError: colocated restore produced a pytree structure that does not match the caller template structure. Got: PyTreeDef({'opt_state': ...

FIXES: b/530279521

Tests

Tested on internal cluster (details in b/529621972#comment6)

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/common/checkpointing.py	60.00%	6 Missing ⚠️

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