Add hapi.text and corresponding unit test. (#24457)

* Add hapi.text and corresponding unit test.
test=develop

* Remove hapi.text apis' reuse parameter args for coverage.
test=develop

* Fix TransformerCell and TransformerBeamSearchDecoder example codes.
test=develop

* Fix example codes in hapi.text.
test=develop

* Add some apis in hapi.text into example code white list.
test=develop

* Fix example code of DynamicDecode in hapi.text.
text=develop

* Rename Model.self as model in test_text.py
test=develop

Author: guoshengCS

python/paddle/incubate/hapi/__init__.py

@@ -22,6 +22,7 @@
 from . import datasets
 from . import distributed
 from . import vision
+from . import text
 
 logger.setup_logger()
 
@@ -33,6 +34,7 @@
     'metrics',
     'loss',
     'vision',
+    'text',
 ]
 
 __all__ += model.__all__