text: Title: Error using augmentation
  Body: Hi everyone, 
  
  I use augmentation as follows:
  
  ```
  > ``seq = iaa.Sometimes(0.833, iaa.Sequential([
      iaa.Fliplr(0.5), # horizontal flips
      iaa.Crop(percent=(0, 0.1)), # random crops
      # Small gaussian blur with random sigma between 0 and 0.5.
      # But we only blur about 50% of all images.
      iaa.Sometimes(0.5,
          iaa.GaussianBlur(sigma=(0, 0.5))
      ),
      # Strengthen or weaken the contrast in each image.
      iaa.ContrastNormalization((0.75, 1.5)),
      # Add gaussian noise.
      # For 50% of all images, we sample the noise once per pixel.
      # For the other 50% of all images, we sample the noise per pixel AND
      # channel. This can change the color (not only brightness) of the
      # pixels.
      iaa.AdditiveGaussianNoise(loc=0, scale=(0.0, 0.05*255), per_channel=0.5),
      # Make some images brighter and some darker.
      # In 20% of all cases, we sample the multiplier once per channel,
      # which can end up changing the color of the images.
      iaa.Multiply((0.8, 1.2), per_channel=0.2),
      # Apply affine transformations to each image.
      # Scale/zoom them, translate/move them, rotate them and shear them.
      iaa.Affine(
          scale={"x": (0.8, 1.2), "y": (0.8, 1.2)},
          translate_percent={"x": (-0.2, 0.2), "y": (-0.2, 0.2)},
          rotate=(-25, 25),
          shear=(-8, 8)
      )
  ], random_order=True)) # apply augmenters in random order``
  ```
  
  and the errors happen with the input array, mask:
  
  ```
  ERROR:root:Error processing image {'id': '1281', 'source': 'dataset', 'path': '/content/drive/My Drive/mask rcnn/train_image/1281.jpg', 'annotation': '/content/drive/My Drive/mask rcnn/train_annotation/1281.xml'}
  Traceback (most recent call last):
    File "/content/drive/My Drive/mask rcnn/mrcnn/model.py", line 1709, in data_generator
      use_mini_mask=config.USE_MINI_MASK)
    File "/content/drive/My Drive/mask rcnn/mrcnn/model.py", line 1254, in load_image_gt
      hooks=imgaug.HooksImages(activator=hook))
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 470, in augment_image
      return self.augment_images([image], hooks=hooks)[0]
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 603, in augment_images
      hooks=hooks
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 3386, in _augment_images
      hooks=hooks
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 603, in augment_images
      hooks=hooks
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 2816, in _augment_images
      hooks=hooks
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/meta.py", line 603, in augment_images
      hooks=hooks
    File "/usr/local/lib/python3.6/dist-packages/imgaug/augmenters/size.py", line 795, in _augment_images
      image_cr_pa = ia.imresize_single_image(image_cr_pa, (height, width))
    File "/usr/local/lib/python3.6/dist-packages/imgaug/imgaug.py", line 1289, in imresize_single_image
      rs = imresize_many_images(image[np.newaxis, :, :, :], sizes, interpolation=interpolation)
    File "/usr/local/lib/python3.6/dist-packages/imgaug/imgaug.py", line 1253, in imresize_many_images
      result[i] = result_img
  ValueError: could not broadcast input array from shape (1024,1024,512) into shape (1024,1024,0)
  ```
  
  ```
  `Traceback (most recent call last):
    File "/content/drive/My Drive/mask rcnn/mrcnn/model.py", line 1709, in data_generator
      use_mini_mask=config.USE_MINI_MASK)
    File "/content/drive/My Drive/mask rcnn/mrcnn/model.py", line 1257, in load_image_gt
      assert mask.shape == mask_shape, "Augmentation shouldn't change mask size"
  AssertionError: Augmentation shouldn't change mask size`
  
  ```
  Hope you guys can help me,