Paddle/doc/fluid/design/others/gan_api.md

# Design for GAN

GAN (General Adversarial Net [https://arxiv.org/abs/1406.2661]) is an important model for unsupervised learning and widely used in many areas. 

It applies several important concepts in machine learning system design, including building and running subgraphs, dependency tracing, different optimizers in one executor and so forth.

In our GAN design, we wrap it as a user-friendly easily customized python API to design different models. We take the conditional DC-GAN (Unsupervised Representation Learning with Deep Convolutional Generative Adversarial Networks [https://arxiv.org/abs/1511.06434]) as an example due to its good performance on image generation.

<p align="center">
<img src="./test.dot.png" width = "35%" align="center"/><br/>
Figure 1. The overall running logic of GAN. The black solid arrows indicate the forward pass; the green dashed arrows indicate the backward pass of generator training; the red dashed arrows indicate the backward pass of the discriminator training. The BP pass of the green (red) arrow should only update the parameters in the green (red) boxes. The diamonds indicate the data providers. d\_loss and g\_loss marked in red and green are the two targets we would like to run.
</p>

The operators, layers and functions required/optional to build a GAN demo is summarized in https://github.com/PaddlePaddle/Paddle/issues/4563.

<p align="center">
<img src="./dcgan.png" width = "90%" align="center"/><br/>
Figure 2. Photo borrowed from the original DC-GAN paper.
</p>

## The Conditional-GAN might be a class. 
This design we adopt the popular open source design in https://github.com/carpedm20/DCGAN-tensorflow and https://github.com/rajathkmp/DCGAN. It contains following data structure:

- DCGAN(object): which contains everything required to build a GAN model. It provides following member functions methods as API:

- __init__(...): Initialize hyper-parameters (like conv dimension and so forth), and declare model parameters of discriminator and generator as well.

- generator(z, y=None): Generate a fake image from input noise z. If the label y is provided, the conditional GAN model will be chosen.
Returns a generated image.

- discriminator(image):
Given an image, decide if it is from a real source or a fake one. 
Returns a 0/1 binary label.

- build_model(self):
build the whole GAN model, define training loss for both generator and discrimator.

## Discussion on Engine Functions required to build GAN
- Trace the tensor and variable dependency in the engine executor. (Very critical, otherwise GAN can'be be trained correctly)
- Different optimizers responsible for optimizing different loss.

To be more detailed, we introduce our design of DCGAN as following:

### Class member Function: Initializer
- Set up hyper-parameters, including condtional dimension, noise dimension, batch size and so forth.
- Declare and define all the model variables. All the discriminator parameters are included in the list self.theta_D and all the generator parameters are included in the list self.theta_G.
```python
class DCGAN(object):
  def __init__(self, y_dim=None):
  
    # hyper parameters  
    self.y_dim = y_dim # conditional gan or not
    self.batch_size = 100
    self.z_dim = z_dim # input noise dimension

    # define parameters of discriminators
    self.D_W0 = pd.Variable(shape=[3,3, 1, 128], data=pd.gaussian_normal_randomizer())
    self.D_b0 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
    self.D_W1 = pd.Variable(shape=[784, 128], data=pd.gaussian_normal_randomizer())
    self.D_b1 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
    self.D_W2 = pd.Varialble(np.random.rand(128, 1))
    self.D_b2 = pd.Variable(np.zeros(128))
    self.theta_D = [self.D_W0, self.D_b0, self.D_W1, self.D_b1, self.D_W2, self.D_b2]

    # define parameters of generators
    self.G_W0 = pd.Variable(shape=[784, 128], data=pd.gaussian_normal_randomizer())
    self.G_b0 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
    self.G_W1 = pd.Variable(shape=[784, 128], data=pd.gaussian_normal_randomizer())
    self.G_b1 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
    self.G_W2 = pd.Varialble(np.random.rand(128, 1))
    self.G_b2 = pd.Variable(np.zeros(128))
    self.theta_G = [self.G_W0, self.G_b0, self.G_W1, self.G_b1, self.G_W2, self.G_b2]
```

### Class member Function: Generator
- Given a noisy input z, returns a fake image.
- Concatenation, batch-norm, FC operations required;
- Deconv layer required, which is missing now...
```python
class DCGAN(object):
  def generator(self, z, y = None):
    # input z: the random noise
    # input y: input data label (optional)
    # output G_im: generated fake images
    
    if not self.y_dim:
      z = pd.layer.concat(1, [z, y])
      
    G_h0 = pd.layer.fc(z, self.G_w0, self.G_b0)
    G_h0_bn = pd.layer.batch_norm(G_h0)
    G_h0_relu = pd.layer.relu(G_h0_bn)
    
    G_h1 = pd.layer.deconv(G_h0_relu, self.G_w1, self.G_b1)
    G_h1_bn = pd.layer.batch_norm(G_h1)
    G_h1_relu = pd.layer.relu(G_h1_bn)
    
    G_h2 = pd.layer.deconv(G_h1_relu, self.G_W2, self.G_b2))
    G_im = pd.layer.tanh(G_im)
    return G_im
```

### Class member function: Discriminator
- Given a noisy input z, returns a fake image.
- Concatenation, Convolution, batch-norm, FC, Leaky-ReLU operations required;
```python
class DCGAN(object):
  def discriminator(self, image):
    # input image: either generated images or real ones
    # output D_h2: binary logit of the label

    D_h0 = pd.layer.conv2d(image, w=self.D_w0, b=self.D_b0)
    D_h0_bn = pd.layer.batchnorm(h0)
    D_h0_relu = pd.layer.lrelu(h0_bn)
    
    D_h1 = pd.layer.conv2d(D_h0_relu, w=self.D_w1, b=self.D_b1)
    D_h1_bn = pd.layer.batchnorm(D_h1)
    D_h1_relu = pd.layer.lrelu(D_h1_bn)
    
    D_h2 = pd.layer.fc(D_h1_relu, w=self.D_w2, b=self.D_b2)
    return D_h2
```

### Class member function: Build the model
- Define data readers as placeholders to hold the data;
- Build generator and discriminators;
- Define two training losses for discriminator and generator, respectively. 
If we have execution dependency engine to back-trace all tensors, the module building our GAN model will be like this:
```python
class DCGAN(object):
  def build_model(self):
    if self.y_dim:
        self.y = pd.data(pd.float32, [self.batch_size, self.y_dim])
    self.images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
    self.faked_images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
    self.z = pd.data(tf.float32, [None, self.z_size])
    
    # step 1: generate images by generator, classify real/fake images with discriminator
    if self.y_dim: # if conditional GAN, includes label
        self.G = self.generator(self.z, self.y)
        self.D_t = self.discriminator(self.images)
        # generated fake images
        self.sampled = self.sampler(self.z, self.y)
        self.D_f = self.discriminator(self.G)
    else: # original version of GAN
        self.G = self.generator(self.z)
        self.D_t = self.discriminator(self.images)
        # generate fake images
        self.sampled = self.sampler(self.z)
        self.D_f = self.discriminator(self.images)
    
    # step 2: define the two losses
    self.d_loss_real = pd.reduce_mean(pd.cross_entropy(self.D_t, np.ones(self.batch_size))
    self.d_loss_fake = pd.reduce_mean(pd.cross_entropy(self.D_f, np.zeros(self.batch_size))
    self.d_loss = self.d_loss_real + self.d_loss_fake
    
    self.g_loss = pd.reduce_mean(pd.cross_entropy(self.D_f, np.ones(self.batch_szie))
```

If we do not have dependency engine but blocks, the module building our GAN model will be like this:
```python
class DCGAN(object):
  def build_model(self, default_block):
    # input data in the default block
    if self.y_dim:
        self.y = pd.data(pd.float32, [self.batch_size, self.y_dim])
    self.images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
    # self.faked_images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
    self.z = pd.data(tf.float32, [None, self.z_size])

    # step 1: generate images by generator, classify real/fake images with discriminator
    with pd.default_block().g_block():
      if self.y_dim: # if conditional GAN, includes label
        self.G = self.generator(self.z, self.y)
        self.D_g = self.discriminator(self.G, self.y)
      else: # original version of GAN
        self.G = self.generator(self.z)
        self.D_g = self.discriminator(self.G, self.y)
      self.g_loss = pd.reduce_mean(pd.cross_entropy(self.D_g, np.ones(self.batch_szie))
    
    with pd.default_block().d_block():
      if self.y_dim: # if conditional GAN, includes label
        self.D_t = self.discriminator(self.images, self.y)
        self.D_f = self.discriminator(self.G, self.y)
      else: # original version of GAN
        self.D_t = self.discriminator(self.images)
        self.D_f = self.discriminator(self.G)

      # step 2: define the two losses
      self.d_loss_real = pd.reduce_mean(pd.cross_entropy(self.D_t, np.ones(self.batch_size))
      self.d_loss_fake = pd.reduce_mean(pd.cross_entropy(self.D_f, np.zeros(self.batch_size))
      self.d_loss = self.d_loss_real + self.d_loss_fake
```
Some small confusion and problems with this design:
- D\_g and D\_f are actually the same thing, but has to be written twice; i.e., if we want to run two sub-graphs conceptually, the same codes have to be written twice if they are shared by the graph.
- Requires ability to create a block anytime, rather than in if-else or rnn only;

## Main function for the demo:
Generally, the user of GAN just need to the following things:
- Define an object as DCGAN class;
- Build the DCGAN model;
- Specify two optimizers for two different losses with respect to different parameters.
```python
# pd for short, should be more concise.
from paddle.v2 as pd
import numpy as np
import logging

if __name__ == "__main__":
    # dcgan class in the default graph/block
    # if we use dependency engine as tensorflow
    # the codes, will be slightly different like:
    # dcgan = DCGAN()
    # dcgan.build_model()
    with pd.block() as def_block:
      dcgan = DCGAN()
      dcgan.build_model(def_block)

    # load mnist data
    data_X, data_y = self.load_mnist()
    
    # Two subgraphs required!!!
    with pd.block().d_block():
      d_optim = pd.train.Adam(lr = .001, beta= .1)
      d_step = d_optim.minimize(dcgan.d_loss, dcgan.theta_D)
    with pd.block.g_block():
      g_optim = pd.train.Adam(lr = .001, beta= .1)
      g_step = pd.minimize(dcgan.g_loss, dcgan.theta_G)

    # executor
    sess = pd.executor()
    
    # training
    for epoch in xrange(10000):
      for batch_id in range(N / batch_size):
        idx = ...
        # sample a batch
        batch_im, batch_label = data_X[idx:idx+batch_size], data_y[idx:idx+batch_size]
        # sample z
        batch_z = np.random.uniform(-1., 1., [batch_size, z_dim])

        if batch_id % 2 == 0:
          sess.run(d_step, 
                   feed_dict = {dcgan.images: batch_im,
                                dcgan.y: batch_label,
                                dcgan.z: batch_z})
        else:
          sess.run(g_step,
                   feed_dict = {dcgan.z: batch_z})
```

# More thinking about dependency engine v.s. block design:
- What if we just want to run an intermediate result? Do we need to run the whole block/graph?
- Should we call eval() to get the fake images in the first stage? And then train the discriminator in the second stage?