Image normalize operator - GPU support, 3D/4D inputs (apache#13802)

* CPU version of normalize operator is working and unit test added

* Add GPU implementation and tests

* Working GPU normalize transforms

* Add default values, fix imports, fix documentation

* Add backward implmentation for image normalize

* Add tests for backward pass

* Move back operators to its original files

* Add review comments

* Add 4D example

* Make infer type generic

* Fix inline function build error

* make functions as inline to avoid multiple definition conflict across cc and cu

* Fix build errors

* Fix failing GPU tests

python/mxnet/gluon/data/vision/transforms.py

@@ -135,7 +135,7 @@ def hybrid_forward(self, F, x):
 
 
 class Normalize(HybridBlock):
-    """Normalize an tensor of shape (C x H x W) with mean and
+    """Normalize an tensor of shape (C x H x W) or (N x C x H x W) with mean and
     standard deviation.
 
     Given mean `(m1, ..., mn)` and std `(s1, ..., sn)` for `n` channels,
@@ -154,12 +154,31 @@ class Normalize(HybridBlock):
 
 
     Inputs:
-        - **data**: input tensor with (C x H x W) shape.
+        - **data**: input tensor with (C x H x W) or (N x C x H x W) shape.
 
     Outputs:
         - **out**: output tensor with the shape as `data`.
+
+    Examples
+    --------
+    >>> transformer = transforms.Normalize(mean=(0, 1, 2), std=(3, 2, 1))
+    >>> image = mx.nd.random.uniform(0, 1, (3, 4, 2))
+    >>> transformer(image)
+    [[[ 0.18293785  0.19761486]
+      [ 0.23839645  0.28142193]
+      [ 0.20092112  0.28598186]
+      [ 0.18162774  0.28241724]]
+     [[-0.2881726  -0.18821815]
+      [-0.17705294 -0.30780914]
+      [-0.2812064  -0.3512327 ]
+      [-0.05411351 -0.4716435 ]]
+     [[-1.0363373  -1.7273437 ]
+      [-1.6165586  -1.5223348 ]
+      [-1.208275   -1.1878313 ]
+      [-1.4711051  -1.5200229 ]]]
+    <NDArray 3x4x2 @cpu(0)>
     """
-    def __init__(self, mean, std):
+    def __init__(self, mean=0.0, std=1.0):
         super(Normalize, self).__init__()
         self._mean = mean
         self._std = std