remove Tracer in en doc, test=develop (#1817)

remove Tracer in release/1.7 en doc

doc/fluid/api/dygraph.rst

@@ -40,5 +40,4 @@ fluid.dygraph
     dygraph/SpectralNorm.rst
     dygraph/to_variable.rst
     dygraph/TracedLayer.rst
-    dygraph/Tracer.rst
     dygraph/TreeConv.rst

doc/fluid/api/dygraph/Tracer.rst

-..  THIS FILE IS GENERATED BY `gen_doc.{py|sh}`
-    !DO NOT EDIT THIS FILE MANUALLY!
-
-.. _api_fluid_dygraph_Tracer:
-
-Tracer
-------
-
-**Note: This API is only avaliable in [Dynamic Graph] mode**
-
-..  autoclass:: paddle.fluid.dygraph.Tracer
-    :members:
-    :noindex:
-