Fix ema's example & fp16 update (#18273)

test=develop, test=document_preview

python/paddle/fluid/optimizer.py

@@ -2473,12 +2473,16 @@ class ExponentialMovingAverage(object):
 
 	.. code-block:: python
 
+	    import numpy
+	    import paddle
 	    import paddle.fluid as fluid
 
 	    data = fluid.layers.data(name='x', shape=[5], dtype='float32')
 	    hidden = fluid.layers.fc(input=data, size=10)
 	    cost = fluid.layers.mean(hidden)
 
+	    test_program = fluid.default_main_program().clone(for_test=True)
+
 	    optimizer = fluid.optimizer.Adam(learning_rate=0.001)
 	    optimizer.minimize(cost)
 
@@ -2486,21 +2490,31 @@ class ExponentialMovingAverage(object):
 	    ema = fluid.optimizer.ExponentialMovingAverage(0.999, thres_steps=global_steps)
 	    ema.update()
 
-	     # pseudo code
-	     for pass_id in range(args.pass_num):
-		 for data in train_reader():
-		     exe.run(fluid.default_main_program()...)
+	    place = fluid.CPUPlace()
+	    exe = fluid.Executor(place)
+	    exe.run(fluid.default_startup_program())
+
+	    for pass_id in range(3):
+		for batch_id in range(6):
+		    data = numpy.random.random(size=(10, 5)).astype('float32')
+		    exe.run(program=fluid.default_main_program(),
+			feed={'x': data}, 
+			fetch_list=[cost.name])
 
 		# usage 1
 		with ema.apply(exe):
-		     for data in test_reader():
-			 exe.run(inference_program...)
+		    data = numpy.random.random(size=(10, 5)).astype('float32')
+		    exe.run(program=test_program,
+			    feed={'x': data}, 
+			    fetch_list=[hidden.name])
+			    
 
 		 # usage 2
 		with ema.apply(exe, need_restore=False):
-		     for data in test_reader():
-			 exe.run(inference_program...)
-                 ...
+		    data = numpy.random.random(size=(10, 5)).astype('float32')
+		    exe.run(program=test_program,
+			    feed={'x': data}, 
+			    fetch_list=[hidden.name])
 		ema.restore(exe)
     """
 
@@ -2590,14 +2604,30 @@ class ExponentialMovingAverage(object):
         Update Exponential Moving Average. Should only call this method in 
         train program.
         """
+        param_master_emas = []
         for param, tmp in self._params_tmps:
             with param.block.program._optimized_guard(
                 [param, tmp]), name_scope('moving_average'):
                 param_ema = self._ema_vars[param.name]
-                ema_t = param_ema * self._decay_var + param * (1 -
-                                                               self._decay_var)
+                if self._ema_vars.has_key(param.name + '.master'):
+                    master_ema = self._ema_vars[param.name + '.master']
+                    param_master_emas.append([param_ema, master_ema])
+                else:
+                    ema_t = param_ema * self._decay_var + param * (
+                        1 - self._decay_var)
                     layers.assign(input=ema_t, output=param_ema)
 
+        # for fp16 params
+        for param_ema, master_ema in param_master_emas:
+            default_main_program().global_block().append_op(
+                type="cast",
+                inputs={"X": master_ema},
+                outputs={"Out": param_ema},
+                attrs={
+                    "in_dtype": master_ema.dtype,
+                    "out_dtype": param_ema.dtype
+                })
+
     @signature_safe_contextmanager
     def apply(self, executor, need_restore=True):
         """