add Decimate and Undecimate layers

onn.py

@@ -358,6 +358,69 @@ class AlphaDropout(Layer):
     def backward(self, dY):
         return dY * self.a * self.mask
 
+class Decimate(Layer):
+    # simple decimaton layer that drops every other sample from the last axis.
+
+    def __init__(self, phase='even'):
+        super().__init__()
+        # phase is the set of samples we keep in the forward pass.
+        assert phase in ('even', 'odd'), phase
+        self.phase = phase
+
+    def make_shape(self, parent):
+        shape = parent.output_shape
+        self.input_shape = shape
+        divy = (shape[-1] + 1) // 2 if self.phase == 'even' else shape[-1] // 2
+        self.output_shape = tuple(list(shape[:-1]) + [divy])
+        self.dX = np.zeros(self.input_shape, dtype=_f)
+
+    def forward(self, X):
+        self.batch_size = X.shape[0]
+        if self.phase == 'even':
+            return X.ravel()[0::2].reshape(self.batch_size, *self.output_shape)
+        elif self.phase == 'odd':
+            return X.ravel()[1::2].reshape(self.batch_size, *self.output_shape)
+
+    def backward(self, dY):
+        assert dY.shape[0] == self.batch_size
+        dX = np.zeros((self.batch_size, *self.input_shape), dtype=_f)
+        if self.phase == 'even':
+            dX.ravel()[0::2] = dY.ravel()
+        elif self.phase == 'odd':
+            dX.ravel()[1::2] = dY.ravel()
+        return dX
+
+class Undecimate(Layer):
+    # reverse operation of Decimate. not quite interpolation.
+
+    def __init__(self, phase='even'):
+        super().__init__()
+        # phase is the set of samples we keep in the backward pass.
+        assert phase in ('even', 'odd'), phase
+        self.phase = phase
+
+    def make_shape(self, parent):
+        shape = parent.output_shape
+        self.input_shape = shape
+        mult = shape[-1] * 2
+        self.output_shape = tuple(list(shape[:-1]) + [mult])
+
+    def forward(self, X):
+        self.batch_size = X.shape[0]
+        Y = np.zeros((self.batch_size, *self.output_shape), dtype=_f)
+        if self.phase == 'even':
+            Y.ravel()[0::2] = X.ravel()
+        elif self.phase == 'odd':
+            Y.ravel()[1::2] = X.ravel()
+        return Y
+
+    def backward(self, dY):
+        assert dY.shape[0] == self.batch_size
+        if self.phase == 'even':
+            return dY.ravel()[0::2].reshape(self.batch_size, *self.input_shape)
+        elif self.phase == 'odd':
+            return dY.ravel()[1::2].reshape(self.batch_size, *self.input_shape)
+
 # Activations {{{2
 
 class Selu(Layer):