fix contrast() transform

See google-research/big_vision#109
Fix suggested by @yeqingli in tensorflow/models#11219 (review)

algorithmic_efficiency/workloads/imagenet_resnet/imagenet_jax/randaugment.py

@@ -128,19 +128,7 @@ def color(image, factor):
 
 def contrast(image, factor):
   """Equivalent of PIL Contrast."""
-  degenerate = tf.image.rgb_to_grayscale(image)
-  # Cast before calling tf.histogram.
-  degenerate = tf.cast(degenerate, tf.int32)
-
-  # Compute the grayscale histogram, then compute the mean pixel value,
-  # and create a constant image size of that value.  Use that as the
-  # blending degenerate target of the original image.
-  hist = tf.histogram_fixed_width(degenerate, [0, 255], nbins=256)
-  mean = tf.reduce_sum(tf.cast(hist, tf.float32)) / 256.0
-  degenerate = tf.ones_like(degenerate, dtype=tf.float32) * mean
-  degenerate = tf.clip_by_value(degenerate, 0.0, 255.0)
-  degenerate = tf.image.grayscale_to_rgb(tf.cast(degenerate, tf.uint8))
-  return blend(degenerate, image, factor)
+  return tf.image.adjust_contrast(image, factor)
 
 
 def brightness(image, factor):