add some comment for api_train_v2 of seqtoseq

Author: jacquesqiao

demo/seqToseq/api_train_v2.py

@@ -73,31 +73,34 @@ def main():
     cost = seqToseq_net_v2(source_dict_dim, target_dict_dim)
     parameters = paddle.parameters.create(cost)
 
+    # define optimize method and trainer
     optimizer = paddle.optimizer.Adam(learning_rate=1e-4)
-
-    def event_handler(event):
-        if isinstance(event, paddle.event.EndIteration):
-            if event.batch_id % 10 == 0:
-                print "Pass %d, Batch %d, Cost %f, %s" % (
-                    event.pass_id, event.batch_id, event.cost, event.metrics)
-
     trainer = paddle.trainer.SGD(cost=cost,
                                  parameters=parameters,
                                  update_equation=optimizer)
 
+    # define data reader
     reader_dict = {
         'source_language_word': 0,
         'target_language_word': 1,
         'target_language_next_word': 2
     }
 
-    trn_reader = paddle.reader.batched(
+    wmt14_reader = paddle.reader.batched(
         paddle.reader.shuffle(
             train_reader("data/pre-wmt14/train/train"), buf_size=8192),
         batch_size=5)
 
+    # define event_handler callback
+    def event_handler(event):
+        if isinstance(event, paddle.event.EndIteration):
+            if event.batch_id % 10 == 0:
+                print "Pass %d, Batch %d, Cost %f, %s" % (
+                    event.pass_id, event.batch_id, event.cost, event.metrics)
+
+    # start to train
     trainer.train(
-        reader=trn_reader,
+        reader=wmt14_reader,
         event_handler=event_handler,
         num_passes=10000,
         reader_dict=reader_dict)