Add config_opencl.json for OpenCL draft backend support

config_opencl.json

@@ -0,0 +1,8 @@
+{
+    "llm_model": "llm.mnn",
+    "llm_weight": "llm.mnn.weight",
+    "backend_type": "opencl",
+    "precision": "low",
+    "memory": "low",
+    "sampler_type": "greedy"
+}