Fix comments of slice_projection, and add unit test of SliceProjection.

paddle/gserver/tests/test_LayerGrad.cpp

@@ -152,6 +152,26 @@ TEST(Projection, identity) {
   }
 }
 
+TEST(Projection, slice) {
+  ProjectionConfig conf;
+  conf.set_type("slice");
+  conf.set_input_size(100);
+  SliceConfig& slice1 = *conf.add_slices();
+  slice1.set_start(10);
+  slice1.set_end(20);
+  SliceConfig& slice2 = *conf.add_slices();
+  slice2.set_start(50);
+  slice2.set_end(70);
+  conf.set_output_size(30);
+  for (auto useGpu : {false, true}) {
+    testProjectionGrad(conf,
+                       INPUT_DATA,
+                       /* parameterSize */ 0,
+                       /* batchSize */ 100,
+                       useGpu);
+  }
+}
+
 TEST(Projection, scaling) {
   ProjectionConfig conf;
   conf.set_type("scaling");

python/paddle/trainer_config_helpers/layers.py

@@ -539,11 +539,11 @@ def identity_projection(input, offset=None, size=None):
 
 def slice_projection(input, slices):
     """
-    slice_projection can get multiple outputs, and each output is a slice
-    of the input.
+    slice_projection can slice the input value into multiple parts,
+    and then select some of them to merge into a new output.
 
     .. math::
-       output[i] = input.slice(slices[i])
+       output = [input.slices()]
 
     The example usage is: