gan tutorial (#462)

* gan tutorial

* formatting fix

* adding pointer to repo; adding navigation link
Co-authored-by: NJeff Rasley <jerasley@microsoft.com>

docs/_data/navigation.yml

@@ -52,6 +52,8 @@ lnav:
         url: /tutorials/azure/
       - title: "CIFAR-10"
         url: /tutorials/cifar-10/
+      - title: "GAN"
+        url: /tutorials/gan/
       - title: "BERT Pre-training"
         url: /tutorials/bert-pretraining/
       - title: "BingBertSQuAD Fine-tuning"

docs/_tutorials/gan.md

+---
+title: "DCGAN Tutorial"
+excerpt: "Train your first GAN model with DeepSpeed!"
+---
+
+If you haven't already, we advise you to first read through the [Getting Started](/getting-started/) guide before stepping through this
+tutorial.
+
+In this tutorial, we will port the DCGAN model to DeepSpeed using custom (user-defined) optimizers and a multi-engine setup!
+
+## Running Original DCGAN
+
+Please go through the [original tutorial](https://pytorch.org/tutorials/beginner/dcgan_faces_tutorial.html) for the Celebrities dataset first using the [original code](https://github.com/pytorch/examples/blob/master/dcgan/main.py). Then run `bash gan_baseline_run.sh`.
+
+
+## Enabling DeepSpeed
+
+The codes may be obtained [here](https://github.com/microsoft/DeepSpeedExamples/tree/master/gan).
+
+### Argument Parsing
+
+The first step to apply DeepSpeed is adding configuration arguments to DCGAN model, using the `deepspeed.add_config_arguments()` function as below.
+
+```python
+import deepspeed
+
+def main():
+    parser = get_argument_parser()
+    parser = deepspeed.add_config_arguments(parser)
+    args = parser.parse_args()
+    train(args)
+```
+
+
+
+### Initialization
+
+We use `deepspeed.initialize` to create two model engines (one for the discriminator network and one for the generator network along with their respective optimizers) as follows:
+
+```python
+    model_engineD, optimizerD, _, _ = deepspeed.initialize(args=args, model=netD, model_parameters=netD.parameters(), optimizer=optimizerD)
+    model_engineG, optimizerG, _, _ = deepspeed.initialize(args=args, model=netG, model_parameters=netG.parameters(), optimizer=optimizerG)
+
+```
+
+Note that DeepSpeed automatically takes care of the distributed training aspect, so we set ngpu=0 to disable the default data parallel mode of pytorch.
+
+### Discriminator Training
+
+We modify the backward for discriminator as follows:
+
+```python
+model_engineD.backward(errD_real)
+model_engineD.backward(errD_fake)
+```
+
+which leads to the inclusion of the gradients due to both real and fake mini-batches in the optimizer update.
+
+### Generator Training
+
+We modify the backward for generator as follows:
+
+```python
+model_engineG.backward(errG)
+```
+
+**Note:** In the case where we use gradient accumulation, backward on the generator would result in accumulation of gradients on the discriminator, due to the tensor dependencies as a result of `errG` being computed from a forward pass through the discriminator; so please set `requires_grad=False` for the `netD` parameters before doing the generator backward.
+
+### Configuration
+
+The next step to use DeepSpeed is to create a configuration JSON file (gan_deepspeed_config.json). This file provides DeepSpeed specific parameters defined by the user, e.g., batch size, optimizer, scheduler and other parameters.
+
+```json
+{
+  "train_batch_size" : 64,
+  "optimizer": {
+    "type": "Adam",
+    "params": {
+      "lr": 0.0002,
+      "betas": [
+        0.5,
+        0.999
+      ],
+      "eps": 1e-8
+    }
+  },
+  "steps_per_print" : 10
+}
+```
+
+
+
+### Run DCGAN Model with DeepSpeed Enabled
+
+To start training the DCGAN model with DeepSpeed, we execute the following command which will use all detected GPUs by default.
+
+```bash
+deepspeed gan_deepspeed_train.py --dataset celeba --cuda --deepspeed_config gan_deepspeed_config.json --tensorboard_path './runs/deepspeed'
+```
+
+## Performance Comparison
+
+We use a total batch size of 64 and perform the training on 16 GPUs for 1 epoch on a DGX-2 node which leads to 3x speed-up. The summary of the the results is given below:
+
+- Baseline total wall clock time for 1 epochs is 393 secs
+
+- Deepspeed total wall clock time for 1 epochs is 128 secs
+
+
+###