move doc/api/v2/fluid to doc/api/fluid

doc/api/fluid/index.rst

+======================
+Fluid
+======================
+
+..  toctree::
+    :maxdepth: 1
+
+    layers.rst
+    data_feeder.rst
+    executor.rst
+    initializer.rst
+    evaluator.rst
+    nets.rst
+    optimizer.rst
+    param_attr.rst
+    profiler.rst
+    regularizer.rst
+    io.rst

doc/api/index_en.rst

@@ -8,4 +8,4 @@ API
     v2/model_configs.rst
     v2/data.rst
     v2/run_logic.rst
-    v2/fluid.rst
+    fluid/index.rst

doc/api/v2/fluid.rst

-======================
-Fluid
-======================
-
-..  toctree::
-    :maxdepth: 1
-
-    fluid/layers.rst
-    fluid/data_feeder.rst
-    fluid/executor.rst
-    fluid/initializer.rst
-    fluid/evaluator.rst
-    fluid/nets.rst
-    fluid/optimizer.rst
-    fluid/param_attr.rst
-    fluid/profiler.rst
-    fluid/regularizer.rst
-    fluid/io.rst