add data provider design doc proposal

doc/design/reader/README.md

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+# Python Data Provider Design Doc
+
+Data provider provides data for training. It will be passed into `paddle.train` as a parameter.
+
+## Data Provider Interface
+
+Data provider is a function with no parameter that creates a iterable (anything can be used in `for x in iterable`):
+
+```
+iterable = data_provider()
+```
+
+Element produced for the iterable should be a **single** entry of data, in format `[column_0_item, column_1_item, ...]`. Each element of the list needs to be supported data type (e.g., numpy 1d array of float32, list of int).
+
+For example, `column_0_item` could be image pixels of format numpy 1d array of float32, and `column_1_item` could be image label of format single int value:
+
+```
+for single_entry in iterable:
+	pixel = entry[0]
+	label = entry[1]
+```
+
+## Usage
+
+data provider, mapping from data provider column to data layer, batch size and number of total pass will be passed into `paddle.train`:
+
+```python
+# two data layer is created:
+image_layer = paddle.layer.data("image", ...)
+label_layer = paddle.layer.data("label", ...)
+
+# ...
+
+paddle.train(paddle.data.mnist, ["image", "label"], 128, 10, ...)
+```
+## Q & A
+
+### Why return only a single entry, but not a mini batch?
+
+If return a mini batch, data provider need to take care of batch size. But batch size is a concept for training, it makes more sense for user to specify batch size as a parameter for `train`.
+
+Concretely, always return a single entry make reusing existing data providers much easier (e.g., if existing data provider return not a single entry but 3 entries, training code will be more complex because it need to handle cases like batch size 2).
+
+### How to create custom data provider
+
+```python
+def image_provider(path):
+	f = open(path)
+	images = numpy.fromfile(
+		f, 'ubyte', count=n * 28 * 28).reshape((n, 28 * 28)).astype('float32')
+	images = images / 255.0 * 2.0 - 1.0
+	labels = numpy.fromfile(l, 'ubyte', count=n).astype("int")
+	for i in xrange(n):
+		yield [images[i, :], labels[i]] # a single entry of data is created each time
+	f.close()
+
+# use python lambda to change image_provier into a function with no parameters.
+provider = lambda : image_provier("/path/to/data/")
+paddle.train(provider, ["image", "label"], ...)
+```
+
+### How is `paddle.train` implemented
+
+An example implementation of paddle.train could be:
+
+```python
+def make_minibatch_generator(reader, minibatch_size):
+    buf = [reader.next() for x in xrange(minibatch_size)]
+    while len(buf) > 0:
+        yield buf
+        buf = [reader.next() for x in xrange(minibatch_size)]
+
+def train(provider, mapping, batch_size, total_pass):
+	for pass_idx in range(total_pass):
+		for mini_batch in make_minibatch_generator(provider(pass_idx)): # this loop will never end in online learning.
+			do_forward_backward(mini_batch, mapping)
+```
+
+This is just an example implementation, more complicated logic like data prefetch could be implemented.