Fix samplewise normalization in ImageDataGenerator (keras-team#8209)

* Fix samplewise normalization in ImageDataGenerator

Normalize pixels uniformly by the image mean and std

* dont specify axes for samplewise image normalization

Author: Noah Stier

keras/preprocessing/image.py

@@ -524,12 +524,10 @@ def standardize(self, x):
             x = self.preprocessing_function(x)
         if self.rescale:
             x *= self.rescale
-        # x is a single image, so it doesn't have image number at index 0
-        img_channel_axis = self.channel_axis - 1
         if self.samplewise_center:
-            x -= np.mean(x, axis=img_channel_axis, keepdims=True)
+            x -= np.mean(x, keepdims=True)
         if self.samplewise_std_normalization:
-            x /= (np.std(x, axis=img_channel_axis, keepdims=True) + 1e-7)
+            x /= np.std(x, keepdims=True) + 1e-7
 
         if self.featurewise_center:
             if self.mean is not None: