fix CUDA HOG for multi GPU systems:

move GPU constants initialization from constructor to compute method

modules/cudaobjdetect/src/hog.cpp

@@ -127,9 +127,6 @@ cv::cuda::HOGDescriptor::HOGDescriptor(Size win_size_, Size block_size_, Size bl
 
     Size cells_per_block = Size(block_size.width / cell_size.width, block_size.height / cell_size.height);
     CV_Assert(cells_per_block == Size(2, 2));
-
-    cv::Size blocks_per_win = numPartsWithin(win_size, block_size, block_stride);
-    hog::set_up_constants(nbins, block_stride.width, block_stride.height, blocks_per_win.width, blocks_per_win.height);
 }
 
 size_t cv::cuda::HOGDescriptor::getDescriptorSize() const
@@ -221,6 +218,9 @@ void cv::cuda::HOGDescriptor::computeGradient(const GpuMat& img, GpuMat& _grad,
 
 void cv::cuda::HOGDescriptor::computeBlockHistograms(const GpuMat& img)
 {
+    cv::Size blocks_per_win = numPartsWithin(win_size, block_size, block_stride);
+    hog::set_up_constants(nbins, block_stride.width, block_stride.height, blocks_per_win.width, blocks_per_win.height);
+
     computeGradient(img, grad, qangle);
 
     size_t block_hist_size = getBlockHistogramSize();