Add ZImageTransformer2DModel and ZImagePipeline to the diffusers library.

Add ZImageTransformer2DModel and ZImagePipeline to the diffusers library.: a task in HF ML Tasksmith (Harbor dataset). Custom attention processor ( ZSingleStreamAttnProcessor ): self-attention only (no cross-attention); applies QK norm (RMSNorm), RoPE via freqs cis , and SDPA. Raises ImportError…

The task

**Custom attention processor** (`ZSingleStreamAttnProcessor`): self-attention only (no cross-attention); applies QK norm (RMSNorm), RoPE via `freqs_cis`, and SDPA. Raises `ImportError` if `torch.nn.functional.scaled_dot_product_attention` is absent.

Part of FineEnvs/HF_ML_Tasksmith.