Fix small bugs with async map

[lhoestq]

helpful for the next PR to enable parallel image/audio/video decoding and make multimodal datasets go brr (e.g. for lerobot)

• fix with_indices
• fix resuming with save_state_dict() / load_state_dict() - omg that wasn't easy
• remove unnecessary decoding in map() to enable parallelism in FormattedExampleIterable later

small bonus: keeping features in batch()

[HuggingFaceDocBuilderDev]

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