Support Grain with Tfrecord in train_tokenizer

[aireenmei]

Description

train_tokenizer.py already supports training with Grain in arrayrecord or parquet formats. Add support for tfrecord

Tests

Add TrainTokenizerFormatTest in tokenizer_test.py to test all 3 formats

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/tokenizer/train_tokenizer.py	83.33%	1 Missing and 1 partial ⚠️

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