Fix the invalid list operation on save_inference_model function.

test=develop

python/paddle/fluid/io.py

@@ -637,8 +637,8 @@ def save_inference_model(dirname,
     if isinstance(target_vars, Variable):
         target_vars = [target_vars]
     elif export_for_deployment:
-        if not (bool(target_vars) and all(
-                isinstance(var, Variable) for var in target_vars)):
+        if not (bool(target_vars) and
+                all(isinstance(var, Variable) for var in target_vars)):
             raise ValueError("'target_vars' should be a list of Variable.")
 
     if main_program is None:
@@ -667,10 +667,15 @@ def save_inference_model(dirname,
     if export_for_deployment:
         main_program = main_program.clone()
         global_block = main_program.global_block()
+        need_to_remove_op_index = []
         for i, op in enumerate(global_block.ops):
             op.desc.set_is_target(False)
             if op.type == "feed" or op.type == "fetch":
-                global_block._remove_op(i)
+                need_to_remove_op_index.append(i)
+
+        for index in need_to_remove_op_index[::-1]:
+            global_block._remove_op(index)
+
         main_program.desc.flush()
 
         main_program = main_program._prune(targets=target_vars)