vulkan: use roundEven for UnaryOp round (#7119)

GLSL round() leaves the direction of halfway cases to the implementation.
MoltenVK rounds them away from zero, while every cpu implementation rounds
half to even, so 0.5 / 2.5 / -0.5 came out as 1 / 3 / -1 on Apple gpus.

Add exact halfway inputs to test_unaryop for op_type 18.

Author: kongxinjun

src/layer/vulkan/shader/unaryop.comp

@@ -46,7 +46,7 @@ void main()
     if (op_type == 16) res = tanh(v);
 #endif
     if (op_type == 17) res = log(v) * afp(0.434294481903);
-    if (op_type == 18) res = round(v);
+    if (op_type == 18) res = roundEven(v);
     if (op_type == 19) res = trunc(v);
     if (op_type == 20) res = sign(v);
     if (op_type == 21) res = exp(v) - afp(1.f);

tests/test_unaryop.cpp

@@ -66,6 +66,16 @@ static int test_unaryop(const ncnn::Mat& _a, int flag = 0)
             a[i] = v;
         }
     }
+#else
+    if (op_type == 18)
+    {
+        // exact halfway values round to even
+        const float halfway[8] = {0.5f, 1.5f, 2.5f, 3.5f, -0.5f, -1.5f, -2.5f, -3.5f};
+        for (int i = 0; i < 8 && i < (int)a.total(); i++)
+        {
+            a[i] = halfway[i];
+        }
+    }
 #endif // __powerpc__
 
     ncnn::ParamDict pd;