Fine-tuning strategy in PyTorch

Introduction

Fine-tuning helps a pre-trained model learn new tasks faster by adjusting it slightly instead of starting from scratch.

You want to teach a model to recognize new types of images but have limited data.

You have a language model trained on general text and want it to work well on medical documents.

You want to improve a speech recognition model for a specific accent.

You want to save time and computing power by building on an existing model.

You want to customize a chatbot to understand your company's products better.

Syntax

PyTorch

import torchvision.models as models
model = models.resnet18(pretrained=True)
for param in model.parameters():
    param.requires_grad = False
model.fc = torch.nn.Linear(model.fc.in_features, num_classes)
optimizer = torch.optim.Adam(model.fc.parameters(), lr=0.001)

# Training loop only updates model.fc parameters

Set requires_grad = False to freeze layers you don't want to change.

Replace the last layer to match your new task's output classes.

Examples

Freeze all layers except the last fully connected layer for a 10-class problem.

PyTorch

import torchvision.models as models
model = models.resnet50(pretrained=True)
for param in model.parameters():
    param.requires_grad = False
model.fc = torch.nn.Linear(model.fc.in_features, 10)  # 10 classes
optimizer = torch.optim.SGD(model.fc.parameters(), lr=0.01)

Fine-tune only the last block of layers (layer4) while freezing the rest.

PyTorch

for name, param in model.named_parameters():
    if 'layer4' in name:
        param.requires_grad = True
    else:
        param.requires_grad = False

Only optimize parameters that require gradients (unfrozen layers).

PyTorch

optimizer = torch.optim.Adam(filter(lambda p: p.requires_grad, model.parameters()), lr=0.0001)

Sample Model

This code fine-tunes only the last layer of a ResNet18 model on a small dummy dataset with 3 classes.

PyTorch

import torch
import torch.nn as nn
import torch.optim as optim
from torchvision import models

# Load pre-trained model
model = models.resnet18(pretrained=True)

# Freeze all layers
for param in model.parameters():
    param.requires_grad = False

# Replace the last layer for 3 classes
num_classes = 3
model.fc = nn.Linear(model.fc.in_features, num_classes)

# Only parameters of final layer will be updated
optimizer = optim.Adam(model.fc.parameters(), lr=0.001)

# Dummy input and target
inputs = torch.randn(5, 3, 224, 224)
targets = torch.tensor([0, 1, 2, 1, 0])

# Loss function
criterion = nn.CrossEntropyLoss()

# Training step
model.train()
optimizer.zero_grad()
outputs = model(inputs)
loss = criterion(outputs, targets)
loss.backward()
optimizer.step()

# Print loss and predictions
print(f"Loss: {loss.item():.4f}")
_, preds = torch.max(outputs, 1)
print(f"Predictions: {preds.tolist()}")

Important Notes

Freezing layers helps keep learned features and reduces training time.

Fine-tuning too many layers with little data can cause overfitting.

Adjust learning rate carefully; often smaller rates work better for fine-tuning.

Summary

Fine-tuning adjusts a pre-trained model to a new task by training some layers.

Freeze layers you want to keep unchanged by setting requires_grad = False.

Replace the last layer to match your new task's output classes and train only it or a few layers.

Practice

1. What is the main purpose of fine-tuning a pre-trained PyTorch model?

easy

A. To adjust the model to perform well on a new task by training some layers

B. To train the model from scratch on a large dataset

C. To reduce the model size by removing layers

D. To convert the model to a different programming language

Solution

1. Step 1: Understand fine-tuning concept

   Fine-tuning means taking a model already trained on one task and adjusting it to work well on a new task by training some of its layers.

2. Step 2: Compare options

   Only To adjust the model to perform well on a new task by training some layers describes this process correctly. Other options describe unrelated actions.

3. Final Answer:

   To adjust the model to perform well on a new task by training some layers -> Option A

4. Quick Check:

   Fine-tuning = Adjust model layers for new task [OK]

Hint: Fine-tuning means training some layers for a new task [OK]

Common Mistakes:

• Thinking fine-tuning means training from scratch
• Confusing fine-tuning with model compression
• Assuming fine-tuning changes the whole model

2. Which PyTorch code snippet correctly freezes all layers except the last one for fine-tuning?

easy

A. model.freeze_all_layers()
model.unfreeze_last_layer()

B. for param in model.parameters(): param.requires_grad = True
for param in model.fc.parameters(): param.requires_grad = False

C. model.requires_grad = False
model.fc.requires_grad = True

D. for param in model.parameters(): param.requires_grad = False
for param in model.fc.parameters(): param.requires_grad = True

Solution

1. Step 1: Understand freezing layers in PyTorch

   Setting param.requires_grad = False freezes a layer so it won't update during training.

2. Step 2: Analyze code snippets

   for param in model.parameters(): param.requires_grad = False for param in model.fc.parameters(): param.requires_grad = True freezes all parameters first, then unfreezes only the last layer (model.fc). The other options reverse or misuse this logic or use non-existent methods.

3. Final Answer:

   for param in model.parameters(): param.requires_grad = False for param in model.fc.parameters(): param.requires_grad = True -> Option D

4. Quick Check:

   Freeze all, unfreeze last layer = for param in model.parameters(): param.requires_grad = False for param in model.fc.parameters(): param.requires_grad = True [OK]

Hint: Freeze all with requires_grad=False, then unfreeze last layer [OK]

Common Mistakes:

• Setting requires_grad True for all layers by mistake
• Using non-existent PyTorch methods
• Forgetting to unfreeze the last layer

3. Given this PyTorch code for fine-tuning, what will be the output of print(sum(p.requires_grad for p in model.parameters()))?

for param in model.parameters():
    param.requires_grad = False
for param in model.classifier.parameters():
    param.requires_grad = True
print(sum(p.requires_grad for p in model.parameters()))

medium

A. Number of all model parameters

B. Number of parameters in model.classifier

C. Zero

D. Raises an error

Solution

1. Step 1: Understand requires_grad flags

   All parameters are first frozen (requires_grad=False). Then only parameters in model.classifier are unfrozen (requires_grad=True).

2. Step 2: Calculate sum of requires_grad

   Summing p.requires_grad counts how many parameters are trainable. Since only model.classifier parameters are True, the sum equals their count.

3. Final Answer:

   Number of parameters in model.classifier -> Option B

4. Quick Check:

   Only classifier params require grad = Number of parameters in model.classifier [OK]

Hint: Sum requires_grad counts trainable parameters [OK]

Common Mistakes:

• Assuming all parameters are trainable
• Confusing boolean sum with total parameters
• Expecting an error from this code

4. You tried to fine-tune a model by freezing layers but the training loss does not change. What is the most likely error in your PyTorch code?

medium

A. You used the wrong optimizer

B. You forgot to set model.train() before training

C. You did not set requires_grad = True for any parameters

D. You replaced the last layer with wrong output size

Solution

1. Step 1: Analyze symptom - loss not changing

   If loss stays the same, model parameters are not updating during training.

2. Step 2: Check requires_grad flags

   If all parameters have requires_grad = False, gradients won't be computed and weights won't update, causing no loss change.

3. Final Answer:

   You did not set requires_grad = True for any parameters -> Option C

4. Quick Check:

   No trainable params = no loss change [OK]

Hint: Check requires_grad True for trainable layers [OK]

Common Mistakes:

• Assuming optimizer choice causes no loss change
• Forgetting to call model.train() but blaming loss
• Ignoring requires_grad flags

5. You want to fine-tune a pre-trained ResNet model on a 10-class problem. Which strategy is best to start with?

hard

A. Freeze all layers, replace the final fully connected layer with 10 outputs, and train only this layer

B. Train the entire ResNet model from scratch with 10 output classes

C. Freeze only the first convolutional layer and train the rest

D. Replace the final layer but keep all layers trainable without freezing

Solution

1. Step 1: Understand common fine-tuning approach

   Starting by freezing all layers except the last layer is a common strategy to adapt a pre-trained model to a new task efficiently.

2. Step 2: Evaluate options

   Freeze all layers, replace the final fully connected layer with 10 outputs, and train only this layer matches this approach: freeze all, replace last layer for 10 classes, train only last layer. Other options either train from scratch or do not freeze enough layers, which can be inefficient or unstable.

3. Final Answer:

   Freeze all layers, replace the final fully connected layer with 10 outputs, and train only this layer -> Option A

4. Quick Check:

   Freeze all but last layer for new task [OK]

Hint: Freeze all, replace last layer, train only it first [OK]

Common Mistakes:

• Training entire model from scratch unnecessarily
• Freezing too few layers causing slow training
• Not replacing last layer to match output classes