Fix get_head_size() when config materializes head_dim as 0

[zohaibm-amd]

Summary

Cherry-picks the get_head_size() portion of upstream commit ceae5bcbda (vllm-project/vllm#45219) to fix the nightly DeepSeek-VL2-tiny_VLM and DeepSeek-VL2-tiny_4096 regressions (AIESW-39871).

Root cause

DeepseekVLV2TextConfig materializes a missing head_dim as 0 rather than leaving it unset. The existing guard in get_head_size() only rejected None, so 0 passed straight through as the architecture head size — while the true head size is hidden_size / num_attention_heads = 1280 / 10 = 128.

Confirmed directly against the model config, no GPU needed:

is_deepseek_mla      -> False
BUGGY get_head_size  -> 0
true (hidden/heads)  -> 128

The resulting zero-sized KV cache is what aborts the engine on the first inference request with hipErrorLaunchFailure / CUDA error: unspecified launch failure.

This is a latent bug being exposed, not a stack regression — the guard has only ever tested for None.

Verification

Run on gfx1151 (Strix Halo) with the nightly's own benchmark arguments, before vs after, identical args:

Config	Before	After
DeepSeek-VL2-tiny_4096	exit 1, 0 successful requests, 5x unspecified launch failure	exit 0, 2 successful requests, prefill 22084 tok/s, decode 138 tok/s
DeepSeek-VL2-tiny_VLM	engine dead, 0 tok/s	prefill 5167 tok/s, decode 154 tok/s

Unit test (upstream's, included here) is non-vacuous — it fails assert 0 == 128 against unpatched code and passes with the fix:

tests/config/test_model_arch_config.py::test_head_size_falls_back_when_head_dim_is_zero PASSED

ruff check and ruff format --check both clean.

Scope note

The upstream commit also carries nixl/CI test changes unrelated to this crash. Only the get_head_size() fix and its regression test are taken here, to keep the later upstream merge (AIESW-37860) conflict-free.

DeepSeek-VL2-tiny_VLM still fails its multimodal sanity check after this fix (answers '100.' instead of 'red'), but that check was already failing on the Jun 26 nightly while the benchmark itself passed — a separate, pre-existing issue, tracked independently of this crash.

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