Allow printer layer to print user provided message

For example:
layer.printer(input=[x, y], format="x=%s y=%s")

paddle/gserver/layers/PrintLayer.cpp

@@ -22,10 +22,33 @@ public:
 
   void forward(PassType passType) override {
     Layer::forward(passType);
+    std::vector<std::string> vals;
     for (size_t i = 0; i != inputLayers_.size(); ++i) {
-      getInput(i).printValueString(LOG(INFO),
-                                   "layer=" + inputLayers_[i]->getName() + " ");
+      std::ostringstream s;
+      getInput(i).printValueString(s, "");
+      vals.push_back(s.str());
     }
+    size_t pos = 0;
+    int i = 0;
+    std::ostringstream s;
+    const std::string& format = config_.user_arg();
+    while (true) {
+      size_t pos1 = format.find("%s", pos);
+      if (pos1 == std::string::npos) break;
+      if (i >= vals.size()) {
+        break;
+      }
+      s << format.substr(pos, pos1 - pos) << vals[i];
+      pos = pos1 + 2;
+      ++i;
+    }
+    if (i != inputLayers_.size()) {
+      LOG(ERROR) << "Number of value in the format (" << format
+                 << ") is not same as the number of inputs ("
+                 << inputLayers_.size() << ") at " << getName();
+    }
+    s << format.substr(pos);
+    LOG(INFO) << s.str();
   }
 
   void backward(const UpdateCallback& callback) override {}

python/paddle/trainer/config_parser.py

@@ -1628,8 +1628,14 @@ class SelectiveFCLayer(LayerBase):
 
 @config_layer('print')
 class PrintLayer(LayerBase):
-    def __init__(self, name, inputs):
+    def __init__(self, name, inputs, format=None):
         super(PrintLayer, self).__init__(name, 'print', 0, inputs)
+        if format is None:
+            format = "\n".join([
+                "layer=" + input.input_layer_name + " %s"
+                for input in self.inputs
+            ])
+        self.config.user_arg = format
 
 
 @config_layer('priorbox')

python/paddle/trainer_config_helpers/layers.py

@@ -964,7 +964,7 @@ def fc_layer(input,
 
 
 @wrap_name_default("print")
-def printer_layer(input, name=None):
+def printer_layer(input, format=None, name=None):
     """
     Print the output value of input layers. This layer is useful for debugging.
 
@@ -982,6 +982,7 @@ def printer_layer(input, name=None):
 
     Layer(
         name=name,
+        format=format,
         type=LayerType.PRINT_LAYER,
         inputs=[l.name for l in input], )
     # this layer don't return anything, can not be input of other layer.

python/paddle/trainer_config_helpers/tests/configs/protostr/test_print_layer.protostr

@@ -12,6 +12,7 @@ layers {
   inputs {
     input_layer_name: "input"
   }
+  user_arg: "layer=input %s"
 }
 input_layer_names: "input"
 output_layer_names: "input"