MetadataConfigs drops parquet shards when exported config rows are non-consecutive

[KirtiRamchandani]

Bug

_from_exported_parquet_files_and_dataset_infos() uses itertools.groupby() on unsorted exported_parquet_files.

If the same config appears again after another config, the earlier shard URLs are lost.

Repro

from itertools import groupby
from operator import itemgetter

exported = [
    {"config": "default", "split": "train", "url": ".../train-0000.parquet"},
    {"config": "other", "split": "train", "url": ".../other/train.parquet"},
    {"config": "default", "split": "train", "url": ".../train-0001.parquet"},
]

metadata_configs = {
    config_name: [p["url"] for _, ps in groupby(rows, itemgetter("split")) for p in ps]
    for config_name, rows in groupby(exported, itemgetter("config"))
}
# default keeps only train-0001

Proof (failing before fix)

FAILED - default only had 1 URL instead of 2

Fix

Sort by (config, split) before both groupby() calls.

I'll follow up with a PR + regression test.