fix sdk for NoMoreTrial status (#394)

* fix bug

* add docs

src/sdk/pynni/nni/msg_dispatcher.py

@@ -99,12 +99,11 @@ class MsgDispatcher(MsgDispatcherBase):
         ids = [_create_parameter_id() for _ in range(data)]
         params_list = self.tuner.generate_multiple_parameters(ids)
 
+        for i, _ in enumerate(params_list):
+            send(CommandType.NewTrialJob, _pack_parameter(ids[i], params_list[i]))
         # when parameters is None.
-        if len(params_list) == 0:
+        if len(params_list) < len(ids):
             send(CommandType.NoMoreTrialJobs, _pack_parameter(ids[0], ''))
-        else:
-            for i, _ in enumerate(ids):
-                send(CommandType.NewTrialJob, _pack_parameter(ids[i], params_list[i]))
         return True
 
     def handle_update_search_space(self, data):

src/sdk/pynni/nni/tuner.py

@@ -41,6 +41,8 @@ class Tuner(Recoverable):
         """Returns multiple sets of trial (hyper-)parameters, as iterable of serializable objects.
         Call 'generate_parameters()' by 'count' times by default.
         User code must override either this function or 'generate_parameters()'.
+        If there's no more trial, user should raise nni.NoMoreTrialError exception in generate_parameters().
+        If so, this function will only return sets of trial (hyper-)parameters that have already been collected.
         parameter_id_list: list of int
         """
         result = []