Turn /sizes into a config-level job
We should:
- compute the values for every config independently: `config--sizes`
- compute the dataset-level response `dataset--sizes` each time a `config--sizes` is computed (allowing partial responses: some config responses can be missing or erroneous)
- return the appropriate response on /sizes, depending on the inputs (see https://github.com/huggingface/datasets-server/pull/834)

See #735.
Turn /sizes into a config-level job: We should:
- compute the values for every config independently: `config--sizes`
- compute the dataset-level response `dataset--sizes` each time a `config--sizes` is computed (allowing partial responses: some config responses can be missing or erroneous)
- return the appropriate response on /sizes, depending on the inputs (see https://github.com/huggingface/datasets-server/pull/834)

See #735.