## Introduction

In machine learning, PyTorch’s nn.CrossEntropyLoss is an often-utilized function. It combines nn.LogSoftmax() and nn.NLLLoss() in one single class to compute the cross-entropy loss between the predicted and actual labels. The CrossEntropyLoss function offers an optional parameter called ignore_index that can be used to ignore the loss contribution from some specific classes and is widely used in NLP, where padding of a sequence is necessary. This post delves into the intricacies of this mechanism and how it affects gradients in backpropagation. I was using ignore_index and ran into an issue with nan values after the first optimization step and had to dive deeper into the issue. As with all code debugging nowadays (and writing), I used ChatGPT to assist me in writing this post.

## Context

I was training a GPT based model and the final output is postprocessed with a mask. In a classic GPT language model, you simply softmax over the vocabulary and it outputs the relevant token. However, I wanted to mask out some parts of the vocabulary and thus created a mask with -inf values. For convenience, I also masked out all the irrelevant tokens with -inf that are not included in the ground truth and also set their relevant class to -1 and set ignore_index = -1 accordingly. Here’s a sample mask:

tensor([[0., 0., -inf, -inf, -inf, -inf],
[0., 0., -inf, -inf, -inf, -inf],
[-inf, -inf, -inf, -inf, -inf, -inf],
[-inf, -inf, -inf, -inf, -inf, -inf],
[-inf, -inf, -inf, -inf, -inf, -inf]])


I started training my model and the loss was a reasonable float value on the first step, but became nan in the second step. This prompted my deep dive. The TLDR here is that in the example mask above, you can mask the first two columns with -inf as that’s what you want, but from the third column onwards, mask with 0.

## A Simple Example

We start with a basic example. We first create an input tensor and a target tensor. We enable gradients for the input tensor and set the ignore_index to -1 as an argument for the CrossEntropyLoss. After computing the loss, we perform a backward pass and print the gradients:

import torch
import torch.nn as nn

# Define the input tensor and target tensor

input_tensor = torch.ones(3, 5)
target_tensor = torch.tensor([1, 2, -1]) # Assuming ignore_index is -1

# Create the CrossEntropyLoss criterion with ignore_index

criterion = nn.CrossEntropyLoss(ignore_index=-1)

# Compute the loss

loss = criterion(input_tensor, target_tensor)

# Print the loss

print("Loss:", loss.item())

# Perform the backward pass

loss.backward()


Output:
Loss: 1.6094379425048828
tensor([[0.1000, -0.4000,  0.1000,  0.1000,  0.1000],
[ 0.1000,  0.1000, -0.4000,  0.1000,  0.1000],
[ 0.0000,  0.0000,  0.0000,  0.0000,  0.0000]])


Observe here that the gradients are indeed 0 for the third row, as the target_tensor has -1 as the third element (or second if 0-indexing). Naturally, if the gradient is zero, then you don’t do any update on the weights, so this is what we want.

## What happens when there are -infs?

Using the same example above, we now explore what happens when we set an entire row to -inf. Specifically, this line was added input_tensor[-1] = float('-inf').

import torch
import torch.nn as nn

# Define the input tensor and target tensor
input_tensor = torch.ones(3, 5)
input_tensor[-1] = float('-inf') # Note this is added!
target_tensor = torch.tensor([1, 2, -1])  # Assuming ignore_index is -1

# Create the CrossEntropyLoss criterion with ignore_index
criterion = nn.CrossEntropyLoss(ignore_index=-1)

# Compute the loss
loss = criterion(input_tensor, target_tensor)

# Print the loss
print("Loss:", loss.item())

# Perform the backward pass
loss.backward()


Output:
Loss: 1.6094379425048828
tensor([[0.1000, -0.4000,  0.1000,  0.1000,  0.1000],
[ 0.1000,  0.1000, -0.4000,  0.1000,  0.1000],
[    nan,     nan,     nan,     nan,     nan]])


Observe now that the entire third row contains nan values, which is the cause of our entire problem.

## How can I debug this in reality?

I read the PyTorch forums and the best way to debug nan values in loss is perhaps to start by adding torch.autograd.anomaly_mode.set_detect_anomaly(True) at the top of your script. After adding that, I immediately saw that the error was:

RuntimeError: Function 'LogSoftmaxBackward0' returned nan values in its 0th output.


which suggested that I had an issue with my gradients. That led me to my deep dive and a better understanding of how ignore_index is used and this blog post.