fix(gpt-oss): route packed attention through THD

[akoumpa]

Summary

• Infer GPT-OSS THD/packed attention from cu_seqlens metadata when qkv_format is not explicitly set.
• Preserve singleton-batch packed outputs while passing TE attention qkv_format=thd and clearing the padding mask before backend preprocessing.
• Add GPT-OSS regression coverage for packed metadata routing and TE backend preprocessing arguments.

Root Cause

The packed recipe passed THD metadata through cu_seqlens, but GptOssForCausalLM.forward only treated the input as packed when attn_kwargs["qkv_format"] == "thd". That left TE preprocessing able to route flattened packed tensors through the BSHD path, which produced the runtime assertion from AMINT-28: Queries, keys and values must be 4D tensors when qkv_format='bshd'.

Validation

• python -m pytest tests/unit_tests/models/gpt_oss/test_gptoss_model.py tests/unit_tests/models/gpt_oss/test_gptoss_layers.py tests/unit_tests/attention/test_attention_utils.py -q (57 passed, 16 skipped)
• python -m ruff check nemo_automodel/components/models/gpt_oss/layers.py nemo_automodel/components/models/gpt_oss/model.py tests/unit_tests/models/gpt_oss/test_gptoss_model.py
• python -m ruff format --check nemo_automodel/components/models/gpt_oss/layers.py nemo_automodel/components/models/gpt_oss/model.py tests/unit_tests/models/gpt_oss/test_gptoss_model.py && git diff --check
• Nemo-CI target rerun passed:
  • parent: https://gitlab-master.nvidia.com/dl/JoC/nemo-ci/-/pipelines/59580947
  • Automodel child: https://gitlab-master.nvidia.com/dl/JoC/nemo-ci/-/pipelines/59582381
  • LLM leaf: https://gitlab-master.nvidia.com/dl/JoC/nemo-ci/-/pipelines/59582398
  • target job: https://gitlab-master.nvidia.com/dl/JoC/nemo-ci/-/jobs/373859894

Note: the successful Nemo-CI run used the x86 EOS/H100 lane and INSTALL_FA3=false to bypass unrelated pre-test flash_attn_3 container build failures observed in earlier attempts.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[akoumpa]

/ok to test f915978