[Bugfix][DeepseekV4] Harden compress_ratio fallback for transformers >=4.57

[varjoranta]

Supersedes #42836. Rebased on top of the post-#43004 DeepSeek V4 restructure. Fixes #42741.

Background

#43004 ("Migrate DeepSeek V4 to vllm/models/ [1/N]") deleted vllm/model_executor/models/deepseek_v4.py, so #42836's patch no longer applies. The transformers >=4.57 compat bug from #42741 / #42836 survived the migration and is now in vllm/models/deepseek_v4/nvidia/model.py:857, in the bare crashing form:

self.compress_ratio = max(1, config.compress_ratios[layer_id])

The bug

transformers 4.57 removed compress_ratios from the DeepSeek V4 config. The bare attribute access then raises AttributeError on every layer init. A naive fallback that does _types = getattr(config, "layer_types", []); _types[layer_id] also breaks: if config.layer_types is explicitly None, getattr(...) returns None (not []) and _types[layer_id] raises TypeError; if the list is shorter than layer_id it raises IndexError.

The fix

if hasattr(config, "compress_ratios") and config.compress_ratios is not None:
    self.compress_ratio = max(1, config.compress_ratios[layer_id])
else:
    _rates = getattr(config, "compress_rates", None) or {}
    _types = getattr(config, "layer_types", None) or []
    layer_type = _types[layer_id] if layer_id < len(_types) else None
    self.compress_ratio = max(1, _rates.get(layer_type, 0))

or {} / or [] coerce missing-attr-or-None to the empty container, the bounds check avoids IndexError, and _rates.get(None, 0) is safe and returns 0, so max(1, 0) = 1 is the conservative fallback (matches the existing "compress ratio can't be 0" invariant).

Closes #42741

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[gemini-code-assist]

Code Review

This pull request updates the compress_ratio calculation in the DeepSeek V4 model to support alternative configuration formats where compress_ratios might be absent. The reviewer identified a potential IndexError in the primary logic if the compress_ratios list is shorter than the current layer_id and suggested using getattr for more idiomatic code. A code suggestion was provided to address these issues and improve robustness against different transformers versions.

[gemini-code-assist]

The hardening logic for compress_ratio is incomplete. While the fallback path correctly handles bounds checking for layer_types, the primary path still risks an IndexError if config.compress_ratios is present but shorter than layer_id. Additionally, using getattr is more idiomatic and concise than hasattr combined with a null check.

Furthermore, please note that a similar pattern exists in vllm/model_executor/layers/attention/mla_attention.py:1309 within get_mla_dims(). If transformers >= 4.57 has indeed removed compress_ratios, that function will likely fail to identify DeepSeek V4 models correctly, leading to an AttributeError when it falls through to the V2/V3 logic and attempts to access attributes like kv_lora_rank which are not present in the V4 config. This should be addressed to ensure full compatibility with the newer transformers versions.

Suggested change

	if hasattr(config, "compress_ratios") and config.compress_ratios is not None:
	self.compress_ratio = max(1, config.compress_ratios[layer_id])
	else:
	_rates = getattr(config, "compress_rates", None) or {}
	_types = getattr(config, "layer_types", None) or []
	layer_type = _types[layer_id] if layer_id < len(_types) else None
	self.compress_ratio = max(1, _rates.get(layer_type, 0))
	_ratios = getattr(config, "compress_ratios", None)
	if _ratios is not None and layer_id < len(_ratios):
	self.compress_ratio = max(1, _ratios[layer_id])
	else:
	_rates = getattr(config, "compress_rates", None) or {}
	_types = getattr(config, "layer_types", None) or []
	layer_type = _types[layer_id] if layer_id < len(_types) else None
	self.compress_ratio = max(1, _rates.get(layer_type, 0))

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @varjoranta.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[varjoranta]

Rebased onto upstream/main (HEAD 228bcc436b). The bare config.compress_ratios[layer_id] access moved from nvidia/model.py to vllm/models/deepseek_v4/attention.py:179 via #43149; same bug, new home, patch reapplied at the new location.

Also folded in @gemini-code-assist's HIGH on the previous revision: the primary path now uses getattr(config, "compress_ratios", None) or [] and an explicit if layer_id < len(ratios) guard before indexing, so a present-but-short compress_ratios falls through to the compress_rates path instead of raising IndexError. Fallback path unchanged.