# Differentiable Neural Computer, for Pytorch

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This is an implementation of [Differentiable Neural Computers](http://people.idsia.ch/~rupesh/rnnsymposium2016/slides/graves.pdf), described in the paper [Hybrid computing using a neural network with dynamic external memory, Graves et al.](https://www.nature.com/articles/nature20101)

## Install

```bash
pip install dnc
```

## Usage

**Parameters**:

| Argument | Default | Description |
| --- | --- | --- |
| input_size | None | Size of the input vectors |
| hidden_size | None | Size of hidden units |
| rnn_type | 'lstm' | Type of recurrent cells used in the controller |
| num_layers | 1 | Number of layers of recurrent units in the controller |
| bias | True | Bias |
| batch_first | True | Whether data is fed batch first |
| dropout | 0 | Dropout between layers in the controller |
| bidirectional | False | If the controller is bidirectional (Not yet implemented) |
| nr_cells | 5 | Number of memory cells |
| read_heads | 2 | Number of read heads |
| cell_size | 10 | Size of each memory cell |
| nonlinearity | 'tanh' | If using 'rnn' as `rnn_type`, non-linearity of the RNNs |
| gpu_id | -1 | ID of the GPU, -1 for CPU |
| independent_linears | False | Whether to use independent linear units to derive interface vector |
| share_memory | True | Whether to share memory between controller layers |
| reset_experience | False | Whether to reset memory (This is a parameter for the forward pass) |


Example usage:

```python
from dnc import DNC

rnn = DNC(
  input_size=64,
  hidden_size=128,
  rnn_type='lstm',
  num_layers=4,
  nr_cells=100,
  cell_size=32,
  read_heads=4,
  batch_first=True,
  gpu_id=0
)

(controller_hidden, memory, read_vectors) = (None, None, None)

output, (controller_hidden, memory, read_vectors) = \
  rnn(torch.randn(10, 4, 64), (controller_hidden, memory, read_vectors, reset_experience=True))
```

## Example copy task

The copy task, as descibed in the original paper, is included in the repo.

From the project root:
```bash
python ./tasks/copy_task.py -cuda 0
```

The copy task can be used to debug memory using [Visdom](https://github.com/facebookresearch/visdom).

Additional step required:

```bash
pip install visdom
python -m visdom.server
```

Open http://localhost:8097/ on your browser, and execute the copy task:

```bash
python ./tasks/copy_task.py -cuda 0
```

The visdom dashboard shows memory as a heatmap for batch 0 every `-summarize_freq` iteration:

![Visdom dashboard](https://user-images.githubusercontent.com/144122/32119691-a54ebd86-bb73-11e7-9ffa-4fa720d7a21a.png)


## General noteworthy stuff

1. DNCs converge with Adam and RMSProp learning rules, SGD generally causes them to diverge.
2. Using a large batch size (> 100, recommended 1000) prevents gradients from becoming `NaN`.