AutoencoderKLWan - support grandient_checkpointing

Do you have a plan for supporting gradient checkpointing for AutoencoderKLWan?

Thank you for always working hard for open source :pray::pray:

Start by locating AutoencoderKLWan and reviewing how gradient checkpointing is supported in related components. Clarify the expected behavior and scope, then add support and tests that demonstrate checkpointing works for this autoencoder.

索引モデルが issue の本文から書いたものです。