Fix download_dir for download_manager used without dataset name. #10290

Job	Run time
Check whether we should run tests or not	9s
Notebook tests	4m 55s
HuggingFace Python 3.10 tests	2m 7s
Github API tests	2m 24s
Core TFDS tests / Generate shards	1s
Core TFDS tests / [Python 3.10][1/4]	4m 45s
Core TFDS tests / [Python 3.11][1/4]	4m 46s
Core TFDS tests / [Python 3.12][1/4]	4m 45s
Core TFDS tests / [Python 3.10][2/4]	4m 48s
Core TFDS tests / [Python 3.11][2/4]	4m 41s
Core TFDS tests / [Python 3.12][2/4]	4m 44s
Core TFDS tests / [Python 3.10][3/4]	4m 43s
Core TFDS tests / [Python 3.11][3/4]	4m 44s
Core TFDS tests / [Python 3.12][3/4]	4m 48s
Core TFDS tests / [Python 3.10][4/4]	4m 44s
Core TFDS tests / [Python 3.11][4/4]	4m 44s
Core TFDS tests / [Python 3.12][4/4]	4m 44s
	1h 6m 32s

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