class_docstr: VGG16 image feature extractor.
docstr: Batch of images to image features.
docstr_tok: ["Batch","of","images","to","image","features","."]
code: def forward(self, imgs):
      """Batch of images to image features."""
      img_feats = self.cnn(imgs, [self.cnn_layer_name])[
          self.cnn_layer_name
      ]
      return img_feats
  
imports: import numpy
  import chainer
  from chainer import functions
  from chainer import initializers
  from chainer import links
  from chainer import reporter
  from chainer import Variable
label_desc: Use image node_id 3 for calling the VGG16FeatureExtractor obj's underlying member method code with example usage: obj.forward(imgs) and returns: img_feats