Fix dygraph rnn doc (#24148)

* update dygraph.rnn import

* update dygraph.rnn import

* change unit to cell

* fix math equations

* fix math equations

* fix examples

* remove unused import

* fix examples

python/paddle/fluid/dygraph/__init__.py

@@ -50,6 +50,9 @@ from .static_runner import StaticModelRunner
 from . import dygraph_to_static
 from .dygraph_to_static import ProgramTranslator
 
+from . import rnn
+from .rnn import *
+
 __all__ = []
 __all__ += layers.__all__
 __all__ += base.__all__
@@ -60,4 +63,5 @@ __all__ += checkpoint.__all__
 __all__ += learning_rate_scheduler.__all__
 __all__ += backward_strategy.__all__
 __all__ += jit.__all__
+__all__ += rnn.__all__
 __all__ += ['ProgramTranslator']

python/paddle/fluid/tests/unittests/test_cudnn_grucell.py

@@ -17,7 +17,7 @@ from __future__ import print_function
 import unittest
 import paddle.fluid as fluid
 import paddle.fluid.core as core
-from paddle.fluid.dygraph.rnn import GRUCell
+from paddle.fluid.dygraph import GRUCell
 
 import numpy as np
 

python/paddle/fluid/tests/unittests/test_cudnn_lstmcell.py

@@ -17,7 +17,7 @@ from __future__ import print_function
 import unittest
 import paddle.fluid as fluid
 import paddle.fluid.core as core
-from paddle.fluid.dygraph.rnn import LSTMCell
+from paddle.fluid.dygraph import LSTMCell
 
 import numpy as np