modify gridmask op (#2693)

Author: wangxinxin08

ppdet/data/transform/gridmask_utils.py

@@ -45,7 +45,8 @@ def __call__(self, x, curr_iter):
         self.prob = self.st_prob * min(1, 1.0 * curr_iter / self.upper_iter)
         if np.random.rand() > self.prob:
             return x
-        h, w, _ = x.shape
+        # image should be C, H, W format
+        _, h, w = x.shape
         hh = int(1.5 * h)
         ww = int(1.5 * w)
         d = np.random.randint(2, h)

static/ppdet/data/transform/gridmask_utils.py

@@ -45,7 +45,8 @@ def __call__(self, x, curr_iter):
         self.prob = self.st_prob * min(1, 1.0 * curr_iter / self.upper_iter)
         if np.random.rand() > self.prob:
             return x
-        h, w, _ = x.shape
+        # image should be C, H, W format
+        _, h, w = x.shape
         hh = int(1.5 * h)
         ww = int(1.5 * w)
         d = np.random.randint(2, h)