title: Color mode in Image Data Generator method flow_from_directory
body: <p>I have a question concerning the flow_from_directory method of ImageDataGenerator . My Images are in RGB form , I am using transfer learning and I want to train the model on grayscale images but pre trained model expects 3 channels . There is a method in flow_from_directory color-mode which takes grayscale or RBG . My question is that if I write grayscale here , does Keras automatically converts the RGB images to grayscale or it is something else ?</p>
  <pre><code>IMAGE_SIZE=[224,224]
  resnet = InceptionResNetV2(input_shape=IMAGE_SIZE + [3] , weights='imagenet', include_top=False)
  </code></pre>
  <p>Flow_from_dir method :</p>
  <pre><code>test_set = test_datagen.flow_from_directory(valid_path,
                                              target_size = (224, 224),
                                              batch_size = 32,
                                              class_mode = 'categorical',color_mode='grayscale')
  </code></pre>