Feature/switch program (#5932)

* Unify fluid submodules to fluid module

Change books just use `import fluid`, not submodules

* Remove g_main_program/g_startup_program

Use default_main_program/default_startup_program instead

* Typo

* Add API for switch default program

* Two functions: switch_main_program/switch_startup_program
* A guard: program_guard. Users can use the `with` statement change
  default programs
* Change unittests in `test_layers`

* Fix CI

* Fix CI

* Fix CI

python/paddle/v2/fluid/framework.py

@@ -3,10 +3,12 @@ import collections
 import numpy as np
 from . import core
 import proto.framework_pb2 as framework_pb2
+import contextlib
 
 __all__ = [
     'Block', 'Variable', 'Program', 'Operator', 'default_startup_program',
-    'default_main_program'
+    'default_main_program', 'program_guard', 'switch_startup_program',
+    'switch_main_program'
 ]
 
 
@@ -659,8 +661,83 @@ _startup_program_ = Program()
 
 
 def default_startup_program():
+    """
+    Get default startup program. In startup program, Paddle will initialize
+    parameters, initialize nccl handle, etc.
+    
+    Returns:
+        Program: startup program
+    """
     return _startup_program_
 
 
 def default_main_program():
+    """
+    Get default main program. The main program is used for training or testing.
+    
+    Returns:
+        Program: main program
+    """
     return _main_program_
+
+
+def switch_main_program(program):
+    """
+    Switch the main program to a new program.
+    
+    Args:
+        program(Program): The new main program
+
+    Returns:
+        Program: The previous main program
+    """
+    global _main_program_
+    prev_program = _main_program_
+    _main_program_ = program
+    return prev_program
+
+
+def switch_startup_program(program):
+    """
+    Switch the startup program to a new program 
+    Args:
+        program(Program): The new startup program
+
+    Returns:
+        Program: The previous startup program
+    """
+    global _startup_program_
+    prev_program = _startup_program_
+    _startup_program_ = program
+    return prev_program
+
+
+@contextlib.contextmanager
+def program_guard(main_program, startup_program=None):
+    """
+    Switch program with `with` statement
+    
+    Examples:
+        >>> with program_guard(Program()):
+        >>>   data = fluid.layers.data(...)
+        >>>   hidden = fluid.layers.fc(...)
+        
+    Args:
+        main_program(Program): New main program inside `with` statement
+        startup_program(Program): New startup program inside `with` statement. 
+            None means do not change startup program.
+
+    Returns:
+        None
+    """
+    if not isinstance(main_program, Program):
+        raise TypeError("main_program should be Program")
+    main_program = switch_main_program(main_program)
+    if startup_program is not None:
+        if not isinstance(startup_program, Program):
+            raise TypeError("startup_program should be Program")
+        startup_program = switch_startup_program(startup_program)
+    yield
+    switch_main_program(main_program)
+    if startup_program is not None:
+        switch_startup_program(startup_program)