add embedding dropout

source/sample/transformer/T2TEncoder.cpp

@@ -99,6 +99,10 @@ XTensor AttEncoder::Make(XTensor &input, XTensor &mask, bool skipInputRes, bool
 
     x = embedder.Make(input);
 
+    /* dropout */
+    if(isTraining && dropoutP > 0)
+        x = Dropout(x);
+
     for(int i = 0; i < nlayer; i++){
         XTensor att;
         XTensor ln;