[Cherry-Pick]Set the default value of protocol to 4. (#32904) #33009

    paddle.save paddle.static.save protocol的默认值改为4（原默认值为2）。

    pickle protocol=4相交于protocol=2：
        protocol=4时保存/加载大于4G的单个numpy.ndarray 等。
        protocol=4时保存/加载的速度有明显提升。
        Python2 不支持protocol=4（paddle2.1主要支持Python3，不再考虑Python2）。

    兼容问题：pickle版本（protocol）会写到文件里面，pickle load的时候会自动识别到protocol，paddle2.1（paddle.save pickle默认版本为2）可以加载paddle2.1.1的模型（paddle.save pickle默认版本为4）。

原始PR：#32904

python/paddle/fluid/io.py

@@ -1788,7 +1788,7 @@ def _legacy_save(param_dict, model_path, protocol=2):
 
 
 @static_only
-def save(program, model_path, protocol=2, **configs):
+def save(program, model_path, protocol=4, **configs):
     """
     :api_attr: Static Graph
 
@@ -1802,7 +1802,7 @@ def save(program, model_path, protocol=2, **configs):
         program(Program) : The program to saved.
         model_path(str): the file prefix to save the program. The format is "dirname/file_prefix". If file_prefix is empty str. A exception will be raised
         protocol(int, optional): The protocol version of pickle module must be greater than 1 and less than 5.
-                                 Default: 2
+                                 Default: 4
         configs(dict, optional) : optional keyword arguments.                        
 
     Returns:

python/paddle/framework/io.py

@@ -491,7 +491,7 @@ def _save_binary_var(obj, path):
             format(type(obj)))
 
 
-def save(obj, path, protocol=2, **configs):
+def save(obj, path, protocol=4, **configs):
     '''
     Save an object to the specified path.
     
@@ -512,7 +512,7 @@ def save(obj, path, protocol=2, **configs):
         path(str) : The path of the object to be saved. 
           If saved in the current directory, the input path string will be used as the file name. 
         protocol(int, optional): The protocol version of pickle module must be greater than 1 and less than 5.
-                                 Default: 2
+                                 Default: 4
         **configs(dict, optional): optional keyword arguments. The following options are currently supported:
           use_binary_format(bool): When the saved object is static graph variable, you can specify ``use_binary_for_var``. 
           If True, save the file in the c++ binary format when saving a single static graph variable; otherwise, save it in pickle format.