RemoteIterator skip iter ckpt

[aireenmei]

Description

Checkpointing Grain iterator when using RemoteIterator (for Pathways+colocated python) is not properly implemented yet. Causing this error: https://b.corp.google.com/issues/466407361#comment20. This PR skips data_iterator checkpointing to unblock testing the data loading part of Grain pipeline. Checkpointing model weights is not impacted. Will fix the checkpointing implementation later.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: https://b.corp.google.com/issues/466407361#comment20

Tests

Tested on v5e-32 and verified checkpointing only model weights work log

Checklist

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