feat: enable dit cache for wan dual transformers.

[z-jun03]

Description

This PR adds DiTCache support for the Wan2.2 dual-transformer pipeline.

The implementation:

• isolates cache state for transformer and transformer_2 with separate scopes;
• maintains independent local cache step IDs for both transformers;
• separates normal and CFG cache states;
• passes the request-level boundary_ratio to transformer selection and cache-step accounting;
• integrates DiTCache step/block callbacks with rolling load;
• keeps rolling H2D scheduling consistent when cache skips individual blocks;
• preserves the existing behavior for non-cache and non-rolling-load paths.

These changes are required because Wan2.2 switches between two transformers during denoising. Sharing one cache state or using the global denoising step would cause cache state, warmup, and residual scheduling to become inconsistent between the two transformers.

Related Issues

N/A

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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Suggested title:

feat: add dit cache support for wan dual transformers.