Complicated neural networks, akin to Giant Language Fashions (LLMs), undergo very often from interpretability challenges. One of the necessary causes for such problem is superposition — a phenomenon of the neural community having fewer dimensions than the variety of options it has to signify. For instance, a toy LLM with 2 neurons has to current 6 completely different language options. Because of this, we observe typically {that a} single neuron must activate for a number of options. For a extra detailed rationalization and definition of superposition, please seek advice from my earlier weblog put up: “Superposition: What Makes it Tough to Clarify Neural Community”.
On this weblog put up, we take one step additional: let’s attempt to disentangle some fsuperposed options. I’ll introduce a strategy referred to as Sparse Autoencoder to decompose advanced neural community, particularly LLM into interpretable options, with a toy instance of language options.
A Sparse Autoencoder, by definition, is an Autoencoder with sparsity launched on function within the activations of its hidden layers. With a moderately easy construction and lightweight coaching course of, it goals to decompose a posh neural community and uncover the options in a extra interpretable method and extra comprehensible to people.
Allow us to think about that you’ve a skilled neural community. The autoencoder shouldn’t be a part of the coaching means of the mannequin itself however is as an alternative a post-hoc evaluation software. The unique mannequin has its personal activations, and these activations are collected afterwards after which used as enter knowledge for the sparse autoencoder.
For instance, we suppose that your unique mannequin is a neural community with one hidden layer of 5 neurons. Moreover, you’ve gotten a coaching dataset of 5000 samples. You need to acquire all of the values of the 5-dimensional activation of the hidden layer for all of your 5000 coaching samples, and they’re now the enter to your sparse autoencoder.
The autoencoder then learns a brand new, sparse illustration from these activations. The encoder maps the unique MLP activations into a brand new vector house with increased illustration dimensions. Wanting again at my earlier 5-neuron easy instance, we would take into account to map it right into a vector house with 20 options. Hopefully, we are going to acquire a sparse autoencoder successfully decomposing the unique MLP activations right into a illustration, simpler to interpret and analyze.
Sparsity is a crucial within the autoencoder as a result of it’s mandatory for the autoencoder to “disentangle” options, with extra “freedom” than in a dense, overlapping house.. With out existence of sparsity, the autoencoder will in all probability the autoencoder may simply be taught a trivial compression with none significant options’ formation.
Language mannequin
Allow us to now construct our toy mannequin. I encourage the readers to notice that this mannequin shouldn’t be practical and even a bit foolish in apply however it’s adequate to showcase how we construct sparse autoencoder and seize some options.
Suppose now we’ve got constructed a language mannequin which has one specific hidden layer whose activation has three dimensions. Allow us to suppose additionally that we’ve got the next tokens: “cat,” “joyful cat,” “canine,” “energetic canine,” “not cat,” “not canine,” “robotic,” and “AI assistant” within the coaching dataset they usually have the next activation values.
knowledge = torch.tensor([
# Cat categories
[0.8, 0.3, 0.1, 0.05], # "cat"
[0.82, 0.32, 0.12, 0.06], # "joyful cat" (just like "cat")
# Canine classes
[0.7, 0.2, 0.05, 0.2], # "canine"
[0.75, 0.3, 0.1, 0.25], # "loyal canine" (just like "canine")# "Not animal" classes
[0.05, 0.9, 0.4, 0.4], # "not cat"
[0.15, 0.85, 0.35, 0.5], # "not canine"
# Robotic and AI assistant (extra distinct in 4D house)
[0.0, 0.7, 0.9, 0.8], # "robotic"
[0.1, 0.6, 0.85, 0.75] # "AI assistant"
], dtype=torch.float32)
Development of autoencoder
We now construct the autoencoder with the next code:
class SparseAutoencoder(nn.Module):
def __init__(self, input_dim, hidden_dim):
tremendous(SparseAutoencoder, self).__init__()
self.encoder = nn.Sequential(
nn.Linear(input_dim, hidden_dim),
nn.ReLU()
)
self.decoder = nn.Sequential(
nn.Linear(hidden_dim, input_dim)
)def ahead(self, x):
encoded = self.encoder(x)
decoded = self.decoder(encoded)
return encoded, decoded
In line with the code above, we see that the encoder has a just one totally linked linear layer, mapping the enter to a hidden illustration with hidden_dim and it then passes to a ReLU activation. The decoder makes use of only one linear layer to reconstruct the enter. Be aware that the absence of ReLU activation within the decoder is intentional for our particular reconstruction case, as a result of the reconstruction may include real-valued and doubtlessly adverse valued knowledge. A ReLU would quite the opposite power the output to remain non-negative, which isn’t fascinating for our reconstruction.
We practice mannequin utilizing the code beneath. Right here, the loss perform has two components: the reconstruction loss, measuring the accuracy of the autoencoder’s reconstruction of the enter knowledge, and a sparsity loss (with weight), which inspires sparsity formulation within the encoder.
# Coaching loop
for epoch in vary(num_epochs):
optimizer.zero_grad()# Ahead go
encoded, decoded = mannequin(knowledge)
# Reconstruction loss
reconstruction_loss = criterion(decoded, knowledge)
# Sparsity penalty (L1 regularization on the encoded options)
sparsity_loss = torch.imply(torch.abs(encoded))
# Complete loss
loss = reconstruction_loss + sparsity_weight * sparsity_loss
# Backward go and optimization
loss.backward()
optimizer.step()
Now we are able to take a look of the outcome. We’ve plotted the encoder’s output worth of every activation of the unique fashions. Recall that the enter tokens are “cat,” “joyful cat,” “canine,” “energetic canine,” “not cat,” “not canine,” “robotic,” and “AI assistant”.
Despite the fact that the unique mannequin was designed with a quite simple structure with none deep consideration, the autoencoder has nonetheless captured significant options of this trivial mannequin. In line with the plot above, we are able to observe a minimum of 4 options that seem like discovered by the encoder.
Give first Characteristic 1 a consideration. This feautre has large activation values on the 4 following tokens: “cat”, “joyful cat”, “canine”, and “energetic canine”. The outcome means that Characteristic 1 may be one thing associated to “animals” or “pets”. Characteristic 2 can also be an fascinating instance, activating on two tokens “robotic” and “AI assistant”. We guess, due to this fact, this function has one thing to do with “synthetic and robotics”, indicating the mannequin’s understanding on technological contexts. Characteristic 3 has activation on 4 tokens: “not cat”, “not canine”, “robotic” and “AI assistant” and that is presumably a function “not an animal”.
Sadly, unique mannequin shouldn’t be an actual mannequin skilled on real-world textual content, however moderately artificially designed with the idea that related tokens have some similarity within the activation vector house. Nevertheless, the outcomes nonetheless present fascinating insights: the sparse autoencoder succeeded in exhibiting some significant, human-friendly options or real-world ideas.
The easy outcome on this weblog put up suggests:, a sparse autoencoder can successfully assist to get high-level, interpretable options from advanced neural networks akin to LLM.
For readers fascinated by a real-world implementation of sparse autoencoders, I like to recommend this article, the place an autoencoder was skilled to interpret an actual giant language mannequin with 512 neurons. This research offers an actual utility of sparse autoencoders within the context of LLM’s interpretability.
Lastly, I present right here this google colab pocket book for my detailed implementation talked about on this article.
