[Bug Fix] Fix image.flip_xx_xx operator where center element not being copied

src/operator/image/image_random-inl.h

@@ -552,7 +552,9 @@ void FlipImpl(const mxnet::TShape &shape, DType *src, DType *dst) {
   for (int i = axis+1; i < shape.ndim(); ++i) tail *= shape[i];
 
   for (int i = 0; i < head; ++i) {
-    for (int j = 0; j < (mid >> 1); ++j) {
+    // if inplace flip, skip the mid point in axis, otherwise copy is required
+    int mid2 = (src == dst) ? mid >> 1 : (mid + 1) >> 1;
+    for (int j = 0; j < mid2; ++j) {
       int idx1 = (i*mid + j) * tail;
       int idx2 = idx1 + (mid-(j << 1)-1) * tail;
       for (int k = 0; k < tail; ++k, ++idx1, ++idx2) {

tests/python/unittest/test_gluon_data_vision.py

@@ -194,18 +194,20 @@ def test_crop_backward(test_nd_arr, TestCase):
 
 @with_seed()
 def test_flip_left_right():
-    data_in = np.random.uniform(0, 255, (300, 300, 3)).astype(dtype=np.uint8)
-    flip_in = data_in[:, ::-1, :]
-    data_trans = nd.image.flip_left_right(nd.array(data_in, dtype='uint8'))
-    assert_almost_equal(flip_in, data_trans.asnumpy())
+    for width in range(3, 301, 7):
+        data_in = np.random.uniform(0, 255, (300, width, 3)).astype(dtype=np.uint8)
+        flip_in = data_in[:, ::-1, :]
+        data_trans = nd.image.flip_left_right(nd.array(data_in, dtype='uint8'))
+        assert_almost_equal(flip_in, data_trans.asnumpy())
 
 
 @with_seed()
 def test_flip_top_bottom():
-    data_in = np.random.uniform(0, 255, (300, 300, 3)).astype(dtype=np.uint8)
-    flip_in = data_in[::-1, :, :]
-    data_trans = nd.image.flip_top_bottom(nd.array(data_in, dtype='uint8'))
-    assert_almost_equal(flip_in, data_trans.asnumpy())
+    for height in range(3, 301, 7):
+        data_in = np.random.uniform(0, 255, (height, 300, 3)).astype(dtype=np.uint8)
+        flip_in = data_in[::-1, :, :]
+        data_trans = nd.image.flip_top_bottom(nd.array(data_in, dtype='uint8'))
+        assert_almost_equal(flip_in, data_trans.asnumpy())
 
 
 @with_seed()
@@ -445,4 +447,3 @@ def test_bbox_crop():
     im_out, im_bbox = transform(img, bbox)
     assert im_out.shape == (3, 3, 3)
     assert im_bbox[0][2] == 3
-