THD training in VLMs

[cuichenx]

What does this PR do ?

Support THD Training in VLMs

TODOs

• Test CP + THD
• Test more models
  • Gemma 3 VL
  • Ministral 3
  • GLM 4.5 v (after this PR)
  • Qwen 2.5 VL (after this PR)
  • Qwen 3 VL (wip in Support qwen3-vl for THD format and CP #1943)

Changelog

• Add specific line by line info of high level changes in this PR.

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Additional Information

Gemma 3 VL: Loss and grad norm are both matching closely between cp1 and cp2, for both BSHD and THD.

MInistral 3: Not sure why grad norm is different between cp1 and cp2. It might be a display issue. I will look into this further. Loss is matching very closely between cp1 and cp2, for both BSHD and THD.

• Related to # (issue)

Summary by CodeRabbit

• New Features

  • Added batch-level sequence packing support for optimized dataset processing
  • Introduced context-parallel distributed training support for Gemma3, Ministral3, GLM, and Qwen vision-language models

• Refactor

  • Updated model forward signatures to support packed sequence parameters across vision-language models

• Tests

  • Added comprehensive test coverage for sequence packing utilities, distributed training configurations, and attention scaling algorithms

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