add yolov4-tiny README and modify yolov4-tiny codes (#445)

* modify src and test codes for yolov4

examples/README.md

@@ -212,6 +212,35 @@ left = 205,right = 576,top = 153,bot = 447
 
 ![](https://s1.ax1x.com/2020/08/28/domYCt.jpg)
 
+## yolov4-tiny目标检测任务 - [tm_yolov4_tiny.cpp](tm_yolov4_tiny.cpp)
+
+使用图片：
+
+![](https://github.com/OAID/Tengine/blob/master/tests/images/ssd_dog.jpg)
+
+```bash
+$ export LD_LIBRARY_PATH=./build/install/lib
+$ ./build/install/bin/tm_yolov4_tiny -m models/yolov4_tiny.tmfile -i images/ssd_dog.jpg -r 1 -t 1
+```
+结果如下：
+
+```bash
+start to run register cpu allocator
+tengine-lite library version: 1.0-dev
+Repeat 1 times, thread 1, avg time 177.72 ms, max_time 177.72 ms, min_time 177.72 ms
+--------------------------------------
+num_detections,10
+16: 74%
+left = 125,right = 327,top = 221,bot = 537
+2: 40%
+7: 84%
+left = 455,right = 703,top = 77,bot = 168
+1: 28%
+left = 56,right = 603,top = 85,bot = 496
+```
+
+![](https://s1.ax1x.com/2020/10/19/0zpvfU.jpg)
+
 ## 人体姿态识别任务 - [tm_openpose.cpp](tm_openpose.cpp)
 
 使用图片：

examples/tm_yolov4_tiny.cpp

@@ -230,7 +230,7 @@ int yolo_num_detections(layer l, float thresh)
                 int obj_index = entry_index(l, b, n * l.w * l.h + i, 4);
                 if (l.output[obj_index] > thresh)
                 {
-                    printf(".....%d -- %f\n",obj_index, l.output[obj_index]);
+                    // printf(".....%d -- %f\n",obj_index, l.output[obj_index]);
                     ++count;
                 }
             }
@@ -277,16 +277,8 @@ void correct_yolo_boxes(detection* dets, int n, int w, int h, int netw, int neth
     int i;
     int new_w = 0;
     int new_h = 0;
-    if ((( float )netw / w) < (( float )neth / h))
-    {
-        new_w = netw;
-        new_h = (h * netw) / w;
-    }
-    else
-    {
-        new_h = neth;
-        new_w = (w * neth) / h;
-    }
+    new_w = netw;
+    new_h = neth;
     for (i = 0; i < n; ++i)
     {
         box b = dets[i].bbox;
@@ -629,8 +621,14 @@ void get_input_data_darknet(const char* image_file, float* input_data, int net_h
     {
         im.data[i] = im.data[i] / 255;
     }
-    sized = letterbox(im, net_w, net_h);
-    memcpy(input_data, sized.data, size * sizeof(float));
+    int ow = net_w;
+    int oh = net_h;
+    sized = resize_image(im, ow, oh);
+    float *resize_data = (float*)sized.data;
+    for (int i = 0; i < size; i++)
+    {
+        input_data[i] = resize_data[i];
+    }
 
     free_image(sized);
     free_image(im);