# Automatic Model Architecture Search for Reading Comprehension
This example shows us how to use Genetic Algorithm to find good model architectures for Reading Comprehension task.

## Search Space
Since attention and recurrent neural network (RNN) module have been proven effective in Reading Comprehension.
We conclude the search space as follow:

1. IDENTITY (Effectively means keep training).
2. INSERT-RNN-LAYER (Inserts a LSTM. Comparing the performance of GRU and LSTM in our experiment, we decided to use LSTM here.)
3. REMOVE-RNN-LAYER
4. INSERT-ATTENTION-LAYER(Inserts a attention layer.)
5. REMOVE-ATTENTION-LAYER
6. ADD-SKIP (Identity between random layers).
7. REMOVE-SKIP (Removes random skip).

![ga-squad-logo](./ga_squad.png)

## New version
Also we have another version which time cost is less and performance is better. We will release soon.

# How to run this example?

## Use downloading script to download data

Execute the following command to download needed files
using the downloading script:

```
chmod +x ./download.sh
./download.sh
```

## Download manually

1. download "dev-v1.1.json" and "train-v1.1.json" in https://rajpurkar.github.io/SQuAD-explorer/

```
wget https://rajpurkar.github.io/SQuAD-explorer/dataset/train-v1.1.json
wget https://rajpurkar.github.io/SQuAD-explorer/dataset/dev-v1.1.json
```

2. download "glove.840B.300d.txt" in https://nlp.stanford.edu/projects/glove/

```
wget http://nlp.stanford.edu/data/glove.840B.300d.zip
unzip glove.840B.300d.zip
```

## Update configuration
Modify `nni/examples/trials/ga_squad/config.yaml`, here is the default configuration:

```
authorName: default
experimentName: example_ga_squad
trialConcurrency: 1
maxExecDuration: 1h
maxTrialNum: 1
#choice: local, remote
trainingServicePlatform: local
#choice: true, false
useAnnotation: false
tuner:
  codeDir: ~/nni/examples/tuners/ga_customer_tuner
  classFileName: customer_tuner.py
  className: CustomerTuner
  classArgs:
    optimize_mode: maximize
trial:
  command: python3 trial.py
  codeDir: ~/nni/examples/trials/ga_squad
  gpuNum: 0
```

In the "trial" part, if you want to use GPU to perform the architecture search, change `gpuNum` from `0` to `1`. You need to increase the `maxTrialNum` and `maxExecDuration`, according to how long you want to wait for the search result.

`trialConcurrency` is the number of trials running concurrently, which is the number of GPUs you want to use, if you are setting `gpuNum` to 1.

## submit this job

```
nnictl create --config ~/nni/examples/trials/ga_squad/config.yaml
```

# Techinal details about the trial

## How does it works
The evolution-algorithm based architecture for question answering has two different parts just like any other examples: the trial and the tuner.

### The trial

The trial has a lot of different files, functions and classes. Here we will only give most of those files a brief introduction:

* `attention.py` contains an implementation for attention mechanism in Tensorflow.
* `data.py` contains functions for data preprocessing.
* `evaluate.py` contains the evaluation script.
* `graph.py` contains the definition of the computation graph.
* `rnn.py` contains an implementation for GRU in Tensorflow.
* `train_model.py` is a wrapper for the whole question answering model.

Among those files, `trial.py` and `graph_to_tf.py` is special.

`graph_to_tf.py` has a function named as `graph_to_network`, here is its skelton code:

```
def graph_to_network(input1,
                     input2,
                     input1_lengths,
                     input2_lengths,
                     graph,
                     dropout_rate,
                     is_training,
                     num_heads=1,
                     rnn_units=256):
    topology = graph.is_topology()
    layers = dict()
    layers_sequence_lengths = dict()
    num_units = input1.get_shape().as_list()[-1]
    layers[0] = input1*tf.sqrt(tf.cast(num_units, tf.float32)) + \
        positional_encoding(input1, scale=False, zero_pad=False)
    layers[1] = input2*tf.sqrt(tf.cast(num_units, tf.float32))
    layers[0] = dropout(layers[0], dropout_rate, is_training)
    layers[1] = dropout(layers[1], dropout_rate, is_training)
    layers_sequence_lengths[0] = input1_lengths
    layers_sequence_lengths[1] = input2_lengths
    for _, topo_i in enumerate(topology):
        if topo_i == '|':
            continue
        if graph.layers[topo_i].graph_type == LayerType.input.value:
            # ......
        elif graph.layers[topo_i].graph_type == LayerType.attention.value:
            # ......
        # More layers to handle
```

As we can see, this function is actually a compiler, that converts the internal model DAG configuration (which will be introduced in the `Model configuration format` section) `graph`, to a Tensorflow computation graph.

```
topology = graph.is_topology()
```

performs topological sorting on the internal graph representation, and the code inside the loop:

```
for _, topo_i in enumerate(topology):
```

performs actually conversion that maps each layer to a part in Tensorflow computation graph.

### The tuner

The tuner is much more simple than the trial. They actually share the same `graph.py`. Besides, the tuner has a `customer_tuner.py`, the most important class in which is `CustomerTuner`:

```
class CustomerTuner(Tuner):
    # ......

    def generate_parameters(self, parameter_id):
        """Returns a set of trial graph config, as a serializable object.
        parameter_id : int
        """
        if len(self.population) <= 0:
            logger.debug("the len of poplution lower than zero.")
            raise Exception('The population is empty')
        pos = -1
        for i in range(len(self.population)):
            if self.population[i].result == None:
                pos = i
                break
        if pos != -1:
            indiv = copy.deepcopy(self.population[pos])
            self.population.pop(pos)
            temp = json.loads(graph_dumps(indiv.config))
        else:
            random.shuffle(self.population)
            if self.population[0].result > self.population[1].result:
                self.population[0] = self.population[1]
            indiv = copy.deepcopy(self.population[0])
            self.population.pop(1)
            indiv.mutation()
            graph = indiv.config
            temp =  json.loads(graph_dumps(graph))
    
    # ......
```

As we can see, the overloaded method `generate_parameters` implements a pretty naive mutation algorithm. The code lines:

```
            if self.population[0].result > self.population[1].result:
                self.population[0] = self.population[1]
            indiv = copy.deepcopy(self.population[0])
```

controls the mutation process. It will always take two random individuals in the population, only keeping and mutating the one with better result.

## Model configuration format

Here is an example of the model configuration, which is passed from the tuner to the trial in the architecture search procedure.

```
{
    "max_layer_num": 50,
    "layers": [
        {
            "input_size": 0,
            "type": 3,
            "output_size": 1,
            "input": [],
            "size": "x",
            "output": [4, 5],
            "is_delete": false
        },
        {
            "input_size": 0,
            "type": 3,
            "output_size": 1,
            "input": [],
            "size": "y",
            "output": [4, 5],
            "is_delete": false
        },
        {
            "input_size": 1,
            "type": 4,
            "output_size": 0,
            "input": [6],
            "size": "x",
            "output": [],
            "is_delete": false
        },
        {
            "input_size": 1,
            "type": 4,
            "output_size": 0,
            "input": [5],
            "size": "y",
            "output": [],
            "is_delete": false
        },
        {"Comment": "More layers will be here for actual graphs."}
    ]
}
```

Every model configuration will has a "layers" section, which is a JSON list of layer definitions. The definition of each layer is also a JSON object, where:

 * `type` is the type of the layer. 0, 1, 2, 3, 4 corresponde to attention, self-attention, RNN, input and output layer respectively.
 * `size` is the length of the output. "x", "y" corresponde to document length / question length, respectively.
 * `input_size` is the number of inputs the layer has.
 * `input` is the indices of layers taken as input of this layer.
 * `output` is the indices of layers use this layer's output as their input.
 * `is_delete` means whether the layer is still available.
