update yolo.py TTA flexibility and extensibility (#506)

* update yolo.py TTA flexibility and extensibility

* Update scale_img()

models/yolo.py

@@ -82,18 +82,19 @@ class Model(nn.Module):
     def forward(self, x, augment=False, profile=False):
         if augment:
             img_size = x.shape[-2:]  # height, width
-            s = [0.83, 0.67]  # scales
-            y = []
-            for i, xi in enumerate((x,
-                                    torch_utils.scale_img(x.flip(3), s[0]),  # flip-lr and scale
-                                    torch_utils.scale_img(x, s[1]),  # scale
-                                    )):
-                # cv2.imwrite('img%g.jpg' % i, 255 * xi[0].numpy().transpose((1, 2, 0))[:, :, ::-1])
-                y.append(self.forward_once(xi)[0])
-
-            y[1][..., :4] /= s[0]  # scale
-            y[1][..., 0] = img_size[1] - y[1][..., 0]  # flip lr
-            y[2][..., :4] /= s[1]  # scale
+            s = [1, 0.83, 0.67]  # scales
+            f = [None, 3, None]  # flips (2-ud, 3-lr)
+            y = []  # outputs
+            for si, fi in zip(s, f):
+                xi = torch_utils.scale_img(x.flip(fi) if fi else x, si)
+                yi = self.forward_once(xi)[0]  # forward
+                # cv2.imwrite('img%g.jpg' % s, 255 * xi[0].numpy().transpose((1, 2, 0))[:, :, ::-1])  # save
+                yi[..., :4] /= si  # de-scale
+                if fi is 2:
+                    yi[..., 1] = img_size[0] - yi[..., 1]  # de-flip ud
+                elif fi is 3:
+                    yi[..., 0] = img_size[1] - yi[..., 0]  # de-flip lr
+                y.append(yi)
             return torch.cat(y, 1), None  # augmented inference, train
         else:
             return self.forward_once(x, profile)  # single-scale inference, train