feat(diffusion): add LongLive WAN training path

[AndysonYs]

What does this PR do ?

Adds the initial offline-latents LongLive WAN MVP requested in #4215, covering clean-history plus noisy-target temporal chunks with windowed attention defaults and SP/TP validation.

Changelog

• Add longlive_wan_step registration so WAN recipes can select the LongLive forward step from scripts/training/run_recipe.py.
• Add LongLive WAN chunk selection, target-only loss masking, teacher-forcing mask helpers, and CP/SP partition utilities.
• Add LongLiveWanForwardStep and LongLiveWanFlowMatchingPipeline for clean-history plus noisy-target temporal chunk training.
• Add LongLive WAN 1.3B and 5B SP long-video recipes with explicit sliding-window attention settings to avoid dense [S, S] masks for long sequences.
• Update WAN mock/dataset config paths to support LongLive latent shape overrides used by the long-video recipe.
• Keep the Megatron-Core submodule clean by making explicit dense self-attention masks opt-in only when the decoder supports self_attention_mask.
• Add focused unit tests for LongLive chunking, noising, masking, recipe wiring, and dense-mask/windowed-attention selection.
• Add scripts/validation/wan_sp_tp_tiny_parity.py to verify tiny WAN TP/SP inference parity with exact tensor equality.
• Document the LongLive WAN MVP commands and the 5B SP long-video smoke path in the WAN example README.

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Additional Information

• Related to [feature] LongLiveWan Long-Video Training Recipe #4215.
• This PR implements the initial offline WAN latents/text embeddings target from [feature] LongLiveWan Long-Video Training Recipe #4215; online raw-video VAE encoding with temporal halo exchange remains a future extension.
• Does not add new required or optional dependencies.
• pre-commit run --all-files passed.
• python -m compileall -q scripts/validation/wan_sp_tp_tiny_parity.py src/megatron/bridge/diffusion/models/wan/longlive_wan_step.py src/megatron/bridge/diffusion/models/wan/longlive_wan_utils.py src/megatron/bridge/diffusion/models/wan/wan_model.py src/megatron/bridge/diffusion/recipes/wan/wan.py tests/unit_tests/diffusion/model/wan/test_longlive_wan_step.py tests/unit_tests/diffusion/recipes/wan/test_wan_recipe.py passed.
• Slurm unit job 3244385: 35 passed, 26 warnings in 3.56s on 4x GB200.
• Slurm tiny SP/TP parity job 3244425: strict_equal=True, max_abs=0.00000000e+00.
• Slurm long-video smoke job 3244426: completed 1/1 iteration on 4x GB200 with 0 skipped and 0 NaN iterations.
• Local uv was unavailable in the conda environment, so pre-commit was run directly after installing pre-commit into the existing mb-longlive-wan environment.

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[yaoyu-33]

Could you please check the default LongLive 1.3B recipe? It looks internally inconsistent: longlive_wan_1_3b_pretrain_config() inherits the text-to-video config with context_parallel_size=4 and the WAN provider default qkv_format="thd", but LongLiveWanFlowMatchingPipeline.validate_qkv_format() rejects anything except qkv_format="sbhd".

Please make the default recipe runnable, or mark it as non-runnable and update the README/tests accordingly.