add enable_imperative, disable_imperative alis; test=develop (#24392)

Author: phlrain

python/paddle/fluid/__init__.py

@@ -89,6 +89,8 @@
 from .dygraph.checkpoint import save_dygraph, load_dygraph
 from .dygraph.varbase_patch_methods import monkey_patch_varbase
 Tensor = LoDTensor
+enable_imperative = enable_dygraph
+disable_imperative = disable_dygraph
 
 __all__ = framework.__all__ + executor.__all__ + \
     trainer_desc.__all__ + transpiler.__all__ + \
@@ -104,6 +106,8 @@
         'dygraph',
         'enable_dygraph',
         'disable_dygraph',
+        'enable_imperative',
+        'disable_imperative',
         'transpiler',
         'nets',
         'optimizer',