In this example, we will implement an unconditional version of Glow: Generative Flow with Invertible 1x1 Convolutions. Glow is a normalizing flow, capable of learning a generative model without any supervision. We will use CIFAR10 to train the network, a data set containing 60.000 images from 10 different classes. At the end of this example, you will be able to generate artificial images by randomly sampling from a Gaussian.

A (slightly) modified version of this implementation has been used in the paper Input Complexity and Out-of-distribution Detection with Likelihood-based Generative Models. Please, cite the paper if its content is relevant for your research:

    title={Input Complexity and Out-of-distribution Detection with Likelihood-based Generative Models},
    author={Joan Serrà and David Álvarez and Vicenç Gómez and Olga Slizovskaia and José F. Núñez and Jordi Luque},
    booktitle={International Conference on Learning Representations},

You can check and download the files exposed in this tutorial from our official GitHub repository. You can also download a sample experiment using default hyper-parameters from this link. Notice that this sample experiment is not optimized and consequently it is not intended to be used for comparative results in your paper.

1. File structure

The first step when starting a Skeltorch project is to create the files required to run a project. We will also create a config.schema.json file to validate the configuration parameters used when creating a new experiment:


2. Configuration file

Glow has quite a few hyper-parameters that can be tuned during training. To provide a flexible environment, we will set them using the configuration file inside the model configuration group. Inside our config.default.json file:

  "data": {
    "dataset": "cifar10",
    "image_size": 32,
    "pixel_depth": 8
  "model": {
    "num_blocks": 3,
    "num_flows": 32,
    "squeezing_factor": 2,
    "permutation_type": "conv",
    "coupling_type": "affine",
    "num_filters": 512,
    "kernel_size": 3
  "training": {
    "batch_size": 32,
    "lr": 1e-4,
    "lr_scheduler_patience": 2,
    "early_stopping_patience": 5

Extra: in order to validate it, we create an auxiliary file named config.schema.json. You can check it in our GitHub repository if you are interested in creating them for your own projects.

3. Data class

We will start implementing our own skeltorch.Data class, which is used to handle all data-related tasks. In this example, this class will be extremely simple and will only consist of loading the data sets required to train the model.

import skeltorch
import torchvision.transforms

class GlowData(skeltorch.Data):
    transforms = None

We will only create two splits: one for training and another for validation. As CIFAR10 comes with a default division of training and testing data, we will use this last one as the validation split. Consequently, no actions are required during the creation of an experiment:

def create(self, data_path):

We will use the default implementation of torchvision. By default, this implementation returns PIL.Image objects instead of torch.Tensor. We will load a composition of required transformations (only one in this example, but could be more) inside self.transforms:

def _load_transforms(self):
    self.transforms = torchvision.transforms.Compose([torchvision.transforms.ToTensor()])

We will load them using the method load_datasets(), which is mandatory to implement in any Skeltorch project:

def load_datasets(self, data_path):
    self.datasets['train'] = torchvision.datasets.CIFAR10(
        data_path, train=True, transform=self.transforms, download=True
    self.datasets['validation'] = torchvision.datasets.CIFAR10(
        data_path, train=False, transform=self.transforms, download=True

We will finish our implementation of the data class by extending the load_loaders() method:

def load_loaders(self, data_path, num_workers):
    self.loaders['train'] =
        batch_size=self.experiment.configuration.get('training', 'batch_size'),
    self.loaders['validation'] =
        batch_size=self.experiment.configuration.get('training', 'batch_size'),

4. Runner class: train pipeline

It is time to implement a custom skeltorch.Runner class. This class will handle the implementation of the different pipelines using the data provided by GlowData according to the configuration parameters established by the user:

import numpy as np
import torch
import torch.optim
import skeltorch
from .model import GlowModel

class GlowRunner(skeltorch.Runner):
    scheduler = None

Notice that the torch.nn.Module object associated with the model is stored inside glow/ Check the example files to get a detailed implementation of it. We will start creating a new instance of both the model and the optimizer using Skeltorch default methods:

def init_model(self, device):
    self.model = GlowModel(
        num_blocks=self.experiment.configuration.get('model', 'num_blocks'),
        num_flows=self.experiment.configuration.get('model', 'num_flows'),
        squeezing_factor=self.experiment.configuration.get('model', 'squeezing_factor'),
        permutation_type=self.experiment.configuration.get('model', 'permutation_type'),
        coupling_type=self.experiment.configuration.get('model', 'coupling_type'),
        num_filters=self.experiment.configuration.get('model', 'num_filters'),
        kernel_size=self.experiment.configuration.get('model', 'kernel_size')

def init_optimizer(self, device):
    self.optimizer = torch.optim.Adam(
        lr=self.experiment.configuration.get('training', 'lr')

In both cases, the hyper-parameters are being extracted from the skeltorch.Configuration object of the experiment (self.experiment). We will also initialize a learning rate scheduler using init_others():

def init_others(self, device):
    self.scheduler = torch.optim.lr_scheduler.ReduceLROnPlateau(
        patience=self.experiment.configuration.get('training', 'lr_scheduler_patience')

As the scheduler has a state, we have to save and load it. Skeltorch comes with two default methods for this task: load_states_others() and save_states_others():

def load_states_others(self, checkpoint_data):

def save_states_others(self):
    return {'scheduler': self.scheduler.state_dict()}

Finally, as the learning rate will be reduced dynamically, we would like to have a graphical representation of it. We can create a new Tensorboard plot for this purpose. We will extend the method train_before_epoch_tasks(), which is a hook that runs at the beginning of every epoch. We will keep default behavior by calling super().train_before_epoch_tasks() before appending our own functionalities:

def train_before_epoch_tasks(self, device):
    self.experiment.tbx.add_scalar('lr', self.optimizer.param_groups[0]['lr'], self.counters['epoch'])

We are ready to implement the main method of the training pipeline: the step_train() method. This method receives the data associated with one iteration of the loaders (both training and validation splits) and returns the loss. Skeltorch uses that loss to back-propagate the model and updates its parameters:

def train_step(self, it_data, device):
    x = it_data[0].to(device)
    x = GlowRunner.add_noise(x, self.experiment.configuration.get('data', 'pixel_depth'))
    z, log_det = self.model(x)
    return GlowRunner.compute_loss(z, log_det, self.experiment.configuration.get('data', 'pixel_depth'))

The first step is to move the data to the correct device, stored in device. Notice that while it_data is a tuple containing both the images and the labels, we are only using the images for our project. After adding noise to the images (to simulate a continuous input), we compute the loss using the static method GlowRunner.compute_loss(). Check the runner file (glow/ for a detailed implementation of it.

Finally, we will update the scheduler and call the test pipeline at the end of every epoch by extending the train_after_epoch_tasks() hook. Check the next section for detailed explanation of the behavior of this pipeline:

def train_after_epoch_tasks(self, device):
    self.scheduler.step(self.losses_epoch['validation'][self.counters['epoch']], self.counters['epoch'])
    self.test(None, device)

To avoid unnecessary computation, we will stop the training if the validation loss has not improved for a certain amount of epochs (parameter given in the configuration file). We can easily do that with the train_early_stopping(), which must return a bool representing whether the training should be stopped or not:

def train_early_stopping(self):
    best_epoch = min(self.losses_epoch['validation'], key=self.losses_epoch['validation'].get)
    early_stopping_patience = self.experiment.configuration.get('training', 'early_stopping_patience')
    return True if self.counters['epoch'] - best_epoch > early_stopping_patience else False

5. Runner class: test pipeline

Glow is a generative model, and the best possible test is to make it generate images. To generate images using a trained version of Glow, we have to generate vectors where each position corresponds to a value sampled from a standard Gaussian distribution. We can then reverse the flows to estimate its associated image. If this process does not sound familiar to you, read the original paper for more details:

def test(self, epoch, device):
    # Check if test has a forced epoch to load objects and restore checkpoint
    if epoch is not None and epoch not in self.experiment.checkpoints_get():
        raise ValueError('Epoch {} not found.'.format(epoch))
    elif epoch is not None:
        self.load_states(epoch, device)

    # Log the start of the test'Starting the test of epoch {}...'.format(self.counters['epoch']))

    # Generate random Gaussian z's and reverse the model
    z = torch.randn((5, self.experiment.configuration.get('data', 'image_size') ** 2 * 3)).to(device)
    x = self.model.reverse(z)

    # Save generated images in TensorBoard
    self.experiment.tbx.add_images('samples', x, self.counters['epoch'])

    # Log the end of the test'Random samples stored in TensorBoard')

Notice that the first step of every pipeline is to load object states. In this example, we will assume that they are already loaded if epoch=None.

We generate a batch of 5 random Gaussian vectors of size img_height * img_width * channels = img_size^2 * 3, where we assume squared color images. Finally, the reverse() method of the model is called to obtain their associated images.

We will save the generated images inside TensorBoard using the SummaryWriter object stored inside self.experiment.tbx.

5. Initializing Skeltorch

The last step is to use our custom GlowData and GlowRunner classes to create a Skeltorch project. Inside our glow/ file:

import skeltorch
from .data import GlowData
from .runner import GlowRunner

# Create Skeltorch object
skel = skeltorch.Skeltorch(GlowData(), GlowRunner())

# Run Skeltorch project

6. Running the pipelines

We are ready to run our example. We have implemented the three pipelines, now it is time to execute them. First, we will start by creating a new experiment with the init pipeline:

python -m glow --experiment-name glow_example init --config-path config.default.json --config-schema-file config.schema.json

The next step is to train the model. Do not forget to include --device cuda in case you want to run it in a GPU:

python -m glow --experiment-name glow_example train

We already have our model trained. We have already performed the test of every epoch by calling it inside train_after_epoch_tasks(). In any case, we could run it again by calling:

python -m glow --experiment-name glow_example test --epoch 22

Where --epoch may receive any epoch from which the checkpoint exists.

7. Visualizing the results

Skeltorch comes with two ways of visualizing results. The simplest one is the verbose.log file stored inside every experiment, containing a complete log of everything that has happened since its creation:

Validation Iteration 910 - Loss 3.713
Validation Iteration 920 - Loss 3.738
Validation Iteration 930 - Loss 3.778
Epoch: 3 | Average Training Loss: 3.768 | Average Validation Loss: 3.748
Starting the test of epoch 3...
Random samples stored in TensorBoard
Checkpoint of epoch 3 saved.
Initializing Epoch 4
Train Iteration 4690 - Loss 3.569
Train Iteration 4700 - Loss 3.746
Train Iteration 4710 - Loss 3.731

However, the best way to visualize results is using TensorBoard. You can initialize it by calling:

python -m glow --experiment-name glow_example tensorboard