vision: enable fast_inference for Gemma 4 via vLLM nightly

Add "gemma4" to VLLM_SUPPORTED_VLM so FastLanguageModel(fast_inference=True)
stops rejecting unsloth/gemma-4-E2B-it / gemma-4-26b-a4b-it at the
vision-model arch gate. vLLM nightly (vllm#39291, merged 2026-04-17)
registers Gemma4ForConditionalGeneration as SupportsLoRA; the
unsloth-zoo#603 vLLM patches (patch_gemma4_vllm_lora_support +
patch_gemma4_vllm_k_eq_v_support) wire LoRA + attention_k_eq_v support
on top of that.

Also force compilation_config=0 and enforce_eager=True for any gemma4
variant. vLLM's -O3 backend currently hits "Tried to erase Node size_N
but it still had N users" in _decompose_size_nodes when splitting the
Gemma 4 FX graph (both dense E2B audio path and MoE 26B-A4B language
path). Until the upstream FX splitter grows support for these size()
users, eager execution preserves correctness — bitwise token parity
with HF naive confirmed at 25/25, 4/4, 5/5 on 3 chat prompts × 32
tokens.

unsloth/models/vision.py

@@ -245,6 +245,7 @@ VLLM_SUPPORTED_VLM = [
     "mistral3",
     "qwen3_vl",
     "qwen3_vl_moe",
+    "gemma4",
 ]
 VLLM_NON_LORA_VLM = [
     "mllama",
@@ -1053,6 +1054,16 @@ class FastBaseModel:
                 is_vision_model = is_vlm,
                 fp8_mode = fp8_mode,
             )
+            # why: vLLM's -O3 compile backend (compilation_config=3) hits
+            # _decompose_size_nodes "Tried to erase Node size_N but it still
+            # had N users" on Gemma 4 multimodal models (both dense E2B and
+            # MoE 26B-A4B). Until the upstream FX splitter is fixed, fall
+            # back to compilation_config=0 (no piecewise compile) for gemma4
+            # so fast_inference still loads. Runtime is slower than -O3 but
+            # correctness is preserved.
+            if any(arch == "gemma4" for arch in (model_types or [])):
+                load_vllm_kwargs.setdefault("compilation_config", 0)
+                load_vllm_kwargs.setdefault("enforce_eager", True)
             for allowed_arg in allowed_args:
                 if allowed_arg not in load_vllm_kwargs and allowed_arg in kwargs:
                     load_vllm_kwargs[allowed_arg] = kwargs[allowed_arg]