fix gather op out_of_range_error in Solov2 cpu inference (#4140)

ppdet/modeling/heads/solov2_head.py

@@ -488,6 +488,9 @@ class SOLOv2Head(nn.Layer):
                     fill_value=self.segm_strides[_ind],
                     dtype="int32"))
         strides = paddle.concat(strides)
+        strides = paddle.concat(
+            [strides, paddle.zeros(
+                shape=[1], dtype='int32')])
         strides = paddle.gather(strides, index=inds[:, 0])
 
         # mask encoding.

static/ppdet/modeling/anchor_heads/solov2_head.py

@@ -379,6 +379,9 @@ class SOLOv2Head(object):
                     dtype="float32",
                     value=self.segm_strides[_ind]))
         strides = fluid.layers.concat(strides)
+        strides = fluid.layers.concat(
+            [strides, fluid.layers.zeros(
+                shape=[1], dtype='float32')])
         strides = fluid.layers.gather(strides, index=inds[:, 0])
 
         # mask encoding.