Add grid_sample en doc

doc/fluid/api/nn.rst

@@ -172,3 +172,4 @@ paddle.nn
     nn/AdaptiveAvgPool2d.rst
     nn/AdaptiveAvgPool3d.rst
     nn/layer/activation/Sigmoid.rst
+    nn/functional/vision/grid_sample.rst

doc/fluid/api/nn/functional.rst

@@ -10,3 +10,4 @@ functional
     functional/mse_loss.rst
     functional/adaptive_avg_pool2d.rst
     functional/adaptive_avg_pool3d.rst
+    functional/vision.rst

doc/fluid/api/nn/functional/vision.rst

+==========
+vision
+==========
+
+..  toctree::
+    :maxdepth: 1
+
+    vision/grid_sample.rst

doc/fluid/api/nn/functional/vision/grid_sample.rst

+..  THIS FILE IS GENERATED BY `gen_doc.{py|sh}`
+    !DO NOT EDIT THIS FILE MANUALLY!
+
+.. _api_nn_functional_vision_grid_sample:
+
+grid_sample
+-------------
+
+..  autofunction:: paddle.nn.functional.vision.grid_sample
+    :noindex:
+