correct the LookaheadOptimizer programDesc, test=develop (#25688)

python/paddle/fluid/optimizer.py

@@ -4884,6 +4884,7 @@ class LookaheadOptimizer(object):
                 inputs={"X": fast_var},
                 outputs={"Out": slow_var})
 
+        with framework.program_guard(main_block.program, startup_program):
             # Add Var k to main prog and startup prog
             k = layers.create_global_var(
                 name="lookahead_k",
@@ -4910,9 +4911,11 @@ class LookaheadOptimizer(object):
             layers.increment(x=step, value=1.0, in_place=True)
 
             # lookahead
-        zero_var = layers.fill_constant(shape=[1], dtype='float32', value=0.0)
+            zero_var = layers.fill_constant(
+                shape=[1], dtype='float32', value=0.0)
 
-        one_var = layers.fill_constant(shape=[1], dtype='float32', value=1.0)
+            one_var = layers.fill_constant(
+                shape=[1], dtype='float32', value=1.0)
 
             mod = layers.elementwise_mod(step, k)
             with layers.control_flow.Switch() as switch:
@@ -4923,7 +4926,8 @@ class LookaheadOptimizer(object):
                         tmp_var = layers.elementwise_add(
                             layers.elementwise_mul(fast_var, alpha),
                             layers.elementwise_mul(
-                            slow_var, layers.elementwise_sub(one_var, alpha)))
+                                slow_var,
+                                layers.elementwise_sub(one_var, alpha)))
                         layers.assign(input=tmp_var, output=slow_var)
                         layers.assign(input=tmp_var, output=fast_var)
                 with switch.default():