data_feeder_en (#606)

* data_feeder_en

* Review

* file path

doc/fluid/api_guides/low_level/layers/data_feeder_en.rst

+.. _api_guide_data_feeder_en:
+
+Feed training/inference data with DataFeeder
+########################################################
+
+Fluid provides the :code:`DataFeeder` class, which converts data types such as numpy array into a :code:`LoDTensor` type to feed the training/inference network.
+
+To create a :code:`DataFeeder` object:
+
+.. code-block:: python
+
+    import paddle.fluid as fluid
+
+    image = fluid.layers.data(name='image', shape=[-1, 3, 224, 224], dtype='float32')
+    label = fluid.layers.data(name='label', shape=[-1, 1], dtype='int64')
+    place = fluid.CUDAPlace(0) if fluid.core.is_compiled_with_cuda() else fluid.CPUPlace()
+    feeder = fluid.DataFeeder(feed_list=[image, label], place=place)
+
+The :code:`feed_list` parameter is a list of variables created by :code:`fluid.layers.data()` . 
+The :code:`place` parameter indicates that data such as numpy array passed in from the Python side should be converted to GPU or CPU :code:`LoDTensor`.
+After creating the :code:`DataFeeder` object, the user can call the :code:`feed(iterable)` method to convert :code:`iterable` data given by user into :code:`LoDTensor` .
+
+:code:`iterable` should be a object of Python List or a Tuple type, and each element in :code:`iterable` is a Python List of length N or Tuple type object, where N is the number of :code:`feed_list` variables passed in when the :code:`DataFeeder` object is created.
+
+The concrete format of :code:`iterable` is:
+
+.. code-block:: python
+
+    iterable = [
+        (image_1, label_1),
+        (image_2, label_2),
+        ...
+        (image_n, label_n)
+    ]
+
+:code:`image_i` and :code:`label_i` are both numpy array data. If the dimension of the input data is [1], such as :code:`label_i`,
+you can feed Python int, float, and other types of data. The data types and dimensions of :code:`image_i` and :code:`label_i` are not necessarily
+the same as :code:`dtype` and :code:`shape` specified at :code:`fluid.layers.data()`. :code:`DataFeeder` internally
+performs the conversion of data types and dimensions. If the :code:`lod_level` of the variable in :code:`feed_list` is not zero, in Fluid, the 0th dimension of each row in the dimensionally converted :code:`iterable` will be returned as :code:`LoD` .
+
+Read :ref:`api_fluid_DataFeeder` for specific usage.
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