Fix pure-text AFD model config identity (release/v0.19.1rc1)

[ShwStone]

Purpose

Backport of #174 to release/v0.19.1rc1.

Fix the model configuration regression introduced by #144 and reported in
#172.

get_afd_model_config() copied and replaced hf_config without updating
hf_text_config. For pure-text models, this broke the original identity:

model_config.hf_text_config is model_config.hf_config

vLLM-Ascend consequently treated DeepSeekV2-Lite as a model with a distinct
nested text configuration. Under Attention TP2 with FlashComm1/SP, MLA skipped
the required query/KV gather and passed TP-local x together with full-batch
RoPE cos/sin, causing npu_interleave_rope to fail.

Issue

• Related issue(s): [Bug]: PR #144 breaks pure-text model config identity and causes Async CAM TP2 FlashComm1 RoPE failure #172, Preserve native model registrations for non-AFD workers #144, Fix pure-text AFD model config identity #174
• Closing keyword, only if fully resolved: Closes [Bug]: PR #144 breaks pure-text model config identity and causes Async CAM TP2 FlashComm1 RoPE failure #172

Scope

• In scope:
  • Convert the model config with deepcopy, which preserves aliasing within
    the copied object graph, so the pure-text identity
    hf_text_config is hf_config is retained automatically.
  • Keep a genuinely distinct nested hf_text_config distinct (as a private
    copy).
  • Unit coverage for both configuration layouts.
• Out of scope:
  • Changes to vLLM or vLLM-Ascend.
  • Changes to FlashComm1/SP scheduling or MLA implementation.
  • Changes to Async CAM communication or MoE ubatching.
  • Changes to multimodal model configuration semantics.

Implementation Notes

get_afd_model_config() now deep-copies the ModelConfig and updates the
architecture on the copied hf_config. deepcopy retains internal aliasing,
so for pure-text configs hf_text_config is hf_config still holds on the copy;
a genuinely distinct nested hf_text_config stays a separate object.

Compared with the shallow-copy variant initially proposed in #174, this avoids
hand-maintaining identity relationships if ModelConfig evolves, at the cost
of privatizing all fields of the copied config.

Test Plan

• pytest tests/unit/package/test_package.py (CPU-only; vLLM-gated tests run
  where vLLM is installed).
• GPU/NPU validation: DeepSeekV2-Lite, Attention TP2 with FlashComm1/SP.

Test Result

• Unit tests pass for both configuration layouts (pure-text identity preserved,
  nested text config kept distinct).
• Real-hardware validation passed: the npu_interleave_rope failure under
  Attention TP2 with FlashComm1/SP is resolved and the model runs correctly.

Docs Impact

• Files updated: none.
• If none, reason: behavior-only bug fix; documented via code comments and
  unit tests.

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Essential PR Checklist

• Purpose is clear and linked to public context when possible.
• Scope is bounded.
• Compatibility with vLLM v0.19.1 is considered.
• No changes are made to the vLLM source checkout.
• Plugin-owned classes or explicit dotted class paths are preferred over monkey patches.
• Any compat shim or monkey patch is isolated, idempotent, version-guarded, documented, and tested.
• Imports remain CPU-safe; CUDA-heavy work is delayed or GPU-gated.
• Validation evidence is included, including skipped GPU tests when applicable.
• Documentation impact is stated.

[Copilot]

Pull request overview

This PR backports the fix for a regression in get_afd_model_config() where converting the model config to an AFD architecture could break the pure-text invariant hf_text_config is hf_config, leading to downstream vLLM-Ascend behavior differences and NPU RoPE failures in certain TP2 + FlashComm1/SP paths.

Changes:

• Switch get_afd_model_config() to deepcopy(model_config) before rewriting hf_config.architectures, preserving aliasing inside the copied object graph.
• Add unit coverage for both pure-text configs (identity preserved) and configs with a genuinely distinct nested hf_text_config (kept distinct via private copy).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
afd_plugin/model_executor/models/model_utils.py	Use deepcopy to preserve hf_config/hf_text_config aliasing when producing the AFD-targeting model config.
tests/unit/package/test_package.py	Extend unit tests to assert correct identity/aliasing behavior for pure-text vs. nested text-config layouts.

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