[data] feat: dump train/test example as JSON (#2666)

### What does this PR do?

This PR adds functionality to save one training and one testing example
as JSON files for reference, making it easier to inspect dataset
formatting and preprocessing.
Related to potential future debugging and reproducibility improvements.

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### Test

Manually verified that two files train_example.json and
test_example.json are saved correctly in the specified local_dir.

### API and Usage Example

This change does not alter the public API.

### Design & Code Changes

- Added code to save train_dataset[0] and test_dataset[0] as JSON files
in local_dir

- Helps with quick inspection and reproducibility of dataset inputs

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examples/data_preprocess/math_dataset.py

@@ -16,6 +16,7 @@ Preprocess the MATH-lighteval dataset to parquet format
 """
 
 import argparse
+import json
 import os
 
 import datasets
@@ -74,7 +75,13 @@ if __name__ == "__main__":
 
     train_dataset.to_parquet(os.path.join(local_dir, "train.parquet"))
     test_dataset.to_parquet(os.path.join(local_dir, "test.parquet"))
-
+    # Save one example as JSON for reference
+    example = train_dataset[0]
+    with open(os.path.join(local_dir, "train_example.json"), "w") as f:
+        json.dump(example, f, indent=2)
+    example = test_dataset[0]
+    with open(os.path.join(local_dir, "test_example.json"), "w") as f:
+        json.dump(example, f, indent=2)
     if hdfs_dir is not None:
         makedirs(hdfs_dir)