Fix missing latents_bn_std dtype cast in VAE normalization

[adi776borate]

What does this PR do?

Follow up to #13050

This PR addresses an unintentional dtype/device mismatch during the VAE image encoding step in Flux2 pipelines.

Currently, inside _encode_vae_image, latents_bn_mean is explicitly cast to match image_latents, but latents_bn_std is left in its default buffer state.

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@asomoza
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