add comment and unit test

test=develop

Author: jacquesqiao

python/paddle/fluid/optimizer.py

@@ -249,6 +249,15 @@ def _create_optimization_pass(self,
 
     def _process_distribute_lookuptable(self, param_grads, loss,
                                         startup_program):
+        """
+        Because distribute lookup table only support SGD optimizer for now, not support
+        other optimizer and regularization, so we should find the table parameter out,
+        and avoid to add regularization and other op for it, and add sgd optimize op
+        for it independently.
+        :param param_grads(list((Var, Var))): list of (param, grad) pair.
+        :param loss: the loss variable.
+        :param startup_program: the startup program
+        """
         program = loss.block.program
         table_name = find_distributed_lookup_table(program)
         table_param = None

python/paddle/fluid/tests/unittests/test_dist_transpiler.py

@@ -641,7 +641,7 @@ def transpiler_test_impl(self):
         # 5 save table
         self.assertEqual([op.type for op in pserver1.blocks[5].ops], ["save"])
 
-        trainer, _ = self.get_trainer(config)
+        trainer, trainer_startup = self.get_trainer(config)
         self.assertEqual(len(trainer.blocks), 1)
         ops = [
             'split_ids', 'prefetch', 'merge_ids', 'sequence_pool',
@@ -655,6 +655,16 @@ def transpiler_test_impl(self):
             'recv', 'concat'
         ]
         self.assertEqual([op.type for op in trainer.blocks[0].ops], ops)
+        startup_ops = [
+            'fill_constant', 'fill_constant', 'fill_constant', 'fill_constant',
+            'fill_constant', 'fill_constant', 'fill_constant', 'fill_constant',
+            'fill_constant', 'fill_constant', 'fill_constant', 'fill_constant',
+            'fill_constant', 'fill_constant', 'uniform_random',
+            'uniform_random', 'recv', 'recv', 'recv', 'fetch_barrier', 'concat',
+            'fake_init'
+        ]
+        self.assertEqual([op.type for op in trainer_startup.blocks[0].ops],
+                         startup_ops)
 
 
 class TestDistLookupTableSliceSize(TestDistLookupTableBase):