Fix convert_format when the requested input format is already the def… #10168

Job	Run time
Check whether we should run tests or not	10s
Notebook tests	8m 14s
HuggingFace Python 3.10 tests	2m 39s
Github API tests	2m 47s
Core TFDS tests / Generate shards	1s
Core TFDS tests / [Python 3.10][1/4]	5m 26s
Core TFDS tests / [Python 3.11][1/4]	5m 23s
Core TFDS tests / [Python 3.12][1/4]	5m 52s
Core TFDS tests / [Python 3.10][2/4]	5m 21s
Core TFDS tests / [Python 3.11][2/4]	5m 34s
Core TFDS tests / [Python 3.12][2/4]	5m 52s
Core TFDS tests / [Python 3.10][3/4]	5m 52s
Core TFDS tests / [Python 3.11][3/4]	6m 12s
Core TFDS tests / [Python 3.12][3/4]	5m 59s
Core TFDS tests / [Python 3.10][4/4]	5m 23s
Core TFDS tests / [Python 3.11][4/4]	5m 24s
Core TFDS tests / [Python 3.12][4/4]	6m 2s
	1h 22m 11s

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