ZImageTransformer2D: Only build attention mask if seqlens are not equal

[Beinsezii]

What does this PR do?

Fixes a small performance regression for Z Image Turbo.

Basically just sets attn_mask to None when it would otherwise be all ones, which is always the case for Z Image Turbo where guidance_scale==1 for typical usage.

On an H100 this improves performance by about 4%, using AttentionBackendName._NATIVE_CUDNN.

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Who can review?

@yiyixuxu or @sayakpaul probably

[sayakpaul]

Cc: @JerryWu-code who contributed the model.