optimize gguf dequant, save mem, support Q2_K

use marlin for lm_head, lm_head only calc last token for prefill
extend context window to 19K for DeepSeek-V3/R1 within 24GB VRAM

ktransformers/optimize/optimize_rules/Qwen2-57B-A14B-Instruct.yaml

@@ -15,6 +15,16 @@
       prefill_device: "cuda"
       generate_op: "KLinearMarlin"
       prefill_op: "KLinearTorch"
+- match:
+    name: "^lm_head"
+    class: torch.nn.Linear
+  replace:
+    class: ktransformers.operators.linear.KTransformersLinear
+    kwargs:
+      generate_device: "cuda"
+      prefill_device: "cuda"
+      generate_op: "KLinearMarlin"
+      prefill_op: "KLinearTorch"
 - match:
     name: "^model\\.layers\\..*\\.mlp$"
     class: ktransformers.models.modeling_qwen2_moe.Qwen2MoeSparseMoeBlock