HyperNetworks - HyperLSTM

We have implemented HyperLSTM introduced in paper HyperNetworks, with annotations using PyTorch. This blog post by David Ha gives a good explanation of HyperNetworks.

We have an experiment that trains a HyperLSTM to predict text on Shakespeare dataset. Here’s the link to code: experiment.py

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HyperNetworks use a smaller network to generate weights of a larger network. There are two variants: static hyper-networks and dynamic hyper-networks. Static HyperNetworks have smaller networks that generate weights (kernels) of a convolutional network. Dynamic HyperNetworks generate parameters of a recurrent neural network for each step. This is an implementation of the latter.

Dynamic HyperNetworks

In a RNN the parameters stay constant for each step. Dynamic HyperNetworks generate different parameters for each step. HyperLSTM has the structure of a LSTM but the parameters of each step are changed by a smaller LSTM network.

In the basic form, a Dynamic HyperNetwork has a smaller recurrent network that generates a feature vector corresponding to each parameter tensor of the larger recurrent network. Let’s say the larger network has some parameter $\color{cyan}{W_h}$ the smaller network generates a feature vector $z_h$ and we dynamically compute $\color{cyan}{W_h}$ as a linear transformation of $z_h$. For instance $\color{cyan}{W_h} = \langle W_{hz}, z_h \rangle$ where $W_{hz}$ is a 3-d tensor parameter and $\langle . \rangle$ is a tensor-vector multiplication. $z_h$ is usually a linear transformation of the output of the smaller recurrent network.

Weight scaling instead of computing

Large recurrent networks have large dynamically computed parameters. These are calculated using linear transformation of feature vector $z$. And this transformation requires an even larger weight tensor. That is, when $\color{cyan}{W_h}$ has shape $N_h \times N_h$, $W_{hz}$ will be $N_h \times N_h \times N_z$.

To overcome this, we compute the weight parameters of the recurrent network by dynamically scaling each row of a matrix of same size. where $W_{hd}$ is a $N_h \times N_h$ parameter matrix.

We can further optimize this when we compute $\color{cyan}{W_h} h$, as where $\odot$ stands for element-wise multiplication.

71from typing import Optional, Tuple
73import torch
74from torch import nn
76from labml_helpers.module import Module
77from labml_nn.lstm import LSTMCell

HyperLSTM Cell

For HyperLSTM the smaller network and the larger network both have the LSTM structure. This is defined in Appendix A.2.2 in the paper.

80class HyperLSTMCell(Module):

input_size is the size of the input $x_t$, hidden_size is the size of the LSTM, and hyper_size is the size of the smaller LSTM that alters the weights of the larger outer LSTM. n_z is the size of the feature vectors used to alter the LSTM weights.

We use the output of the smaller LSTM to compute $z_h^{i,f,g,o}$, $z_x^{i,f,g,o}$ and $z_b^{i,f,g,o}$ using linear transformations. We calculate $d_h^{i,f,g,o}(z_h^{i,f,g,o})$, $d_x^{i,f,g,o}(z_x^{i,f,g,o})$, and $d_b^{i,f,g,o}(z_b^{i,f,g,o})$ from these, using linear transformations again. These are then used to scale the rows of weight and bias tensors of the main LSTM.

📝 Since the computation of $z$ and $d$ are two sequential linear transformations these can be combined into a single linear transformation. However we’ve implemented this separately so that it matches with the description in the paper.

88    def __init__(self, input_size: int, hidden_size: int, hyper_size: int, n_z: int):
106        super().__init__()

The input to the hyperLSTM is where $x_t$ is the input and $h_{t-1}$ is the output of the outer LSTM at previous step. So the input size is hidden_size + input_size.

The output of hyperLSTM is $\hat{h}_t$ and $\hat{c}_t$.

119        self.hyper = LSTMCell(hidden_size + input_size, hyper_size, layer_norm=True)

🤔 In the paper it was specified as I feel that it’s a typo.

125        self.z_h = nn.Linear(hyper_size, 4 * n_z)

127        self.z_x = nn.Linear(hyper_size, 4 * n_z)

129        self.z_b = nn.Linear(hyper_size, 4 * n_z, bias=False)

132        d_h = [nn.Linear(n_z, hidden_size, bias=False) for _ in range(4)]
133        self.d_h = nn.ModuleList(d_h)

135        d_x = [nn.Linear(n_z, hidden_size, bias=False) for _ in range(4)]
136        self.d_x = nn.ModuleList(d_x)

138        d_b = [nn.Linear(n_z, hidden_size) for _ in range(4)]
139        self.d_b = nn.ModuleList(d_b)

The weight matrices $W_h^{i,f,g,o}$

142        self.w_h = nn.ParameterList([nn.Parameter(torch.zeros(hidden_size, hidden_size)) for _ in range(4)])

The weight matrices $W_x^{i,f,g,o}$

144        self.w_x = nn.ParameterList([nn.Parameter(torch.zeros(hidden_size, input_size)) for _ in range(4)])

Layer normalization

147        self.layer_norm = nn.ModuleList([nn.LayerNorm(hidden_size) for _ in range(4)])
148        self.layer_norm_c = nn.LayerNorm(hidden_size)
150    def forward(self, x: torch.Tensor,
151                h: torch.Tensor, c: torch.Tensor,
152                h_hat: torch.Tensor, c_hat: torch.Tensor):

159        x_hat = torch.cat((h, x), dim=-1)

161        h_hat, c_hat = self.hyper(x_hat, h_hat, c_hat)

164        z_h = self.z_h(h_hat).chunk(4, dim=-1)

166        z_x = self.z_x(h_hat).chunk(4, dim=-1)

168        z_b = self.z_b(h_hat).chunk(4, dim=-1)

We calculate $i$, $f$, $g$ and $o$ in a loop

171        ifgo = []
172        for i in range(4):

174            d_h = self.d_h[i](z_h[i])

176            d_x = self.d_x[i](z_x[i])

183            y = d_h * torch.einsum('ij,bj->bi', self.w_h[i], h) + \
184                d_x * torch.einsum('ij,bj->bi', self.w_x[i], x) + \
185                self.d_b[i](z_b[i])
187            ifgo.append(self.layer_norm[i](y))

190        i, f, g, o = ifgo

193        c_next = torch.sigmoid(f) * c + torch.sigmoid(i) * torch.tanh(g)

196        h_next = torch.sigmoid(o) * torch.tanh(self.layer_norm_c(c_next))
198        return h_next, c_next, h_hat, c_hat

HyperLSTM module

201class HyperLSTM(Module):

Create a network of n_layers of HyperLSTM.

206    def __init__(self, input_size: int, hidden_size: int, hyper_size: int, n_z: int, n_layers: int):
211        super().__init__()

Store sizes to initialize state

214        self.n_layers = n_layers
215        self.hidden_size = hidden_size
216        self.hyper_size = hyper_size

Create cells for each layer. Note that only the first layer gets the input directly. Rest of the layers get the input from the layer below

220        self.cells = nn.ModuleList([HyperLSTMCell(input_size, hidden_size, hyper_size, n_z)] +
221                                   [HyperLSTMCell(hidden_size, hidden_size, hyper_size, n_z) for _ in
222                                    range(n_layers - 1)])
  • x has shape [n_steps, batch_size, input_size] and
  • state is a tuple of $h, c, \hat{h}, \hat{c}$. $h, c$ have shape [batch_size, hidden_size] and $\hat{h}, \hat{c}$ have shape [batch_size, hyper_size].
224    def forward(self, x: torch.Tensor,
225                state: Optional[Tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]] = None):
232        n_steps, batch_size = x.shape[:2]

Initialize the state with zeros if None

235        if state is None:
236            h = [x.new_zeros(batch_size, self.hidden_size) for _ in range(self.n_layers)]
237            c = [x.new_zeros(batch_size, self.hidden_size) for _ in range(self.n_layers)]
238            h_hat = [x.new_zeros(batch_size, self.hyper_size) for _ in range(self.n_layers)]
239            c_hat = [x.new_zeros(batch_size, self.hyper_size) for _ in range(self.n_layers)]
241        else:
242            (h, c, h_hat, c_hat) = state

Reverse stack the tensors to get the states of each layer

📝 You can just work with the tensor itself but this is easier to debug

246            h, c = list(torch.unbind(h)), list(torch.unbind(c))
247            h_hat, c_hat = list(torch.unbind(h_hat)), list(torch.unbind(c_hat))

Collect the outputs of the final layer at each step

250        out = []
251        for t in range(n_steps):

Input to the first layer is the input itself

253            inp = x[t]

Loop through the layers

255            for layer in range(self.n_layers):

Get the state of the layer

257                h[layer], c[layer], h_hat[layer], c_hat[layer] = \
258                    self.cells[layer](inp, h[layer], c[layer], h_hat[layer], c_hat[layer])

Input to the next layer is the state of this layer

260                inp = h[layer]

Collect the output $h$ of the final layer

262            out.append(h[-1])

Stack the outputs and states

265        out = torch.stack(out)
266        h = torch.stack(h)
267        c = torch.stack(c)
268        h_hat = torch.stack(h_hat)
269        c_hat = torch.stack(c_hat)
272        return out, (h, c, h_hat, c_hat)