feat(data): add PackedTensor preprocessing modes

[rohitrango]

What does this PR do ?

Adds configurable PackedTensor preprocessing and patchifies Nemotron Omni pixels before materialization.

• Replaces the top-level pad_to_max_shape boolean with preprocess_mode and preprocess_kwargs.
• Adds native-resolution patchification into per-image (C_i, P²) blocks, packs them on dimension 0, and returns (1, total_C, P²).
• Carries preprocessing settings through slicing, concatenation, wire transport, and materialization.
• Enables patchification with patch size 16 for supported Nemotron Omni processors.
• Adds NemotronH_Omni_Reasoning_V3Processor to the placeholder-style processor set.

Issues

None.

Usage

PackedTensor(
    pixel_values,
    dim_to_pack=0,
    preprocess_mode="patchify",
    preprocess_kwargs={"patch_dim": 16},
)

Before your PR is "Ready for review"

• Read and followed the contributor guidelines.
• Added or updated unit coverage for preprocessing and patchification.
• Full pre-commit lint suite passed on Slurm job 18281365 (Ruff lint, import sorting, Ruff format, Pyrefly, and config checks).
• Affected unit suite passed on Slurm job 18281365: 90 passed, 1 skipped (megatron.bridge unavailable).
• GRPO and SFT GPU recipe validation passed on Slurm job 18281365.
• Updated relevant inline documentation.

Additional Information

• Stacked on feat: add Energon-owned SFT sequence packing #4105 via the rohit/sft_v2_stage2 base branch. Retarget this PR to main after feat: add Energon-owned SFT sequence packing #4105 merges.

Scripts validated

uv run examples/run_vlm_grpo.py --config examples/configs/recipes/vlm/vlm_grpo-nemotron-omni-30ba3b-clevr-2n8g-megatron-tp8ep8.v1-tq_mooncake.yaml logger.wandb.name=grpo-nemotron-with-patches logger.wandb_enabled=true logger.wandb.project=sft-dev cluster.num_nodes=1

uv run examples/run_sft_v2.py --config examples/configs/recipes/vlm/vlm_sft-nemotron-omni-30ba3b-clevr-1n8g-megatron-tp8ep8-energon.v1.packing.yaml logger.wandb.name=sft-nemotron-with-patches logger.wandb_enabled=true logger.wandb.project=sft-dev sft.max_steps=50

(brown: baseline, green: with patches)

Previous PR and self-review

Previous PR: #4082. A self-review was performed on the previous PR, and the resulting feedback was addressed in this PR.

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