[Refactor][TCPCG] Standardize diffusion eager graph wrappers (7/9)

[Oasis-Git]

This is step 7/9, split from the original TCPCG removal PR #41634.

Depends on #42286 (2/9); merge those dependencies first. This branch targets upstream main, so its diff includes unmerged prerequisites. Review this step alone using the isolated step-7 diff.

Series

1. Relocate shared graph tensor and DSA head-gate helpers — [Refactor][TCPCG] Relocate shared graph tensor and DSA head-gate helpers (1/9) #42285.
2. Add explicit batch support to BCG eager regions — [Refactor][TCPCG] Bind live forward batches in BCG eager calls (2/9) #42286.
3. Retire TCPCG backend selection, configuration, documentation, and tests — [Refactor][TCPCG] Retire backend selection and configuration (3/9) #42287.
4. Consolidate Radix attention eager regions and isolate Inkling-specific behavior — [Refactor][TCPCG] Consolidate attention eager regions and isolate Inkling policy (4/9) #42288.
5. Migrate DSA and DeepSeek eager regions — [Refactor][TCPCG] Migrate DSA and DeepSeek graph breaks to eager methods (5/9) #42289.
6. Migrate MoE and Mamba eager regions — [Refactor][TCPCG] Migrate MoE and Mamba graph execution to eager methods (6/9) #42290.
7. Migrate diffusion eager wrappers — this PR. Depends only on step 2.
8. Remove obsolete runtime contexts, registries, and split-op plumbing.
9. Simplify quantization paths separately because of their torch.compile implications.

The disputed relocation of inactive compiler/reference files is excluded pending a separate decision.

Changes

Migrate diffusion attention and MiniMax H3 wrappers to bare @eager_on_graph and consistently named _eager_ functions. Preserve tuple boxing and the existing graph-dispatch behavior.

This is the independent diffusion slice: it depends only on the eager API introduced in step 2, not the SRT caller migrations in steps 3–6.

Validation

Pre-commit and Python syntax checks passed on the changed files. Diffusion CUDA execution and upstream CI remain to be validated; the local development machine is macOS.

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CI States

Latest PR Test (Base): ❌ Run #37077968419
Latest PR Test (Extra): ❌ Run #37077968211
Latest PR Test (AMD ROCm 10): ❌ Run #37077968500