[Fix] Fix the compilation error on windows (#1657)

Author: zhouzaida

mmcv/ops/csrc/common/cuda/convex_iou_cuda_kernel.cuh

@@ -10,7 +10,7 @@
 
 #define MAXN 100
 #define NMAX 512
-const double EPS = 1E-8;
+__device__ const double EPS = 1E-8;
 
 __device__ inline int sig(double d) { return (d > EPS) - (d < -EPS); }
 

mmcv/ops/csrc/common/cuda/min_area_polygons_cuda.cuh

@@ -9,8 +9,7 @@
 #endif
 
 #define MAXN 20
-const float EPS = 1E-8;
-const float PI = 3.1415926;
+__device__ const float PI = 3.1415926;
 
 struct Point {
   float x, y;