ONNX export in PyTorch - ML Experiment: Train & Evaluate

Experiment - ONNX export

Problem:You have a PyTorch model trained for digit classification. You want to export it to ONNX format to use it in other frameworks or deployment environments.

Current Metrics:Training accuracy: 98%, Validation accuracy: 96%, Model saved only in PyTorch format (.pt)

Issue:The model is only saved in PyTorch format, which limits interoperability with other tools that support ONNX. You need to export the model correctly to ONNX format.

Your Task

Export the given PyTorch model to ONNX format ensuring the exported model can be loaded and run with the same input shape.

Do not retrain the model.

Use the existing trained model weights.

Ensure the ONNX export includes input and output names.

Use a dummy input tensor with the correct shape for export.

Hint 1

Hint 2

Hint 3

Hint 4

Solution

PyTorch

import torch
import torch.nn as nn
import torch.onnx

# Define a simple model (for example purposes)
class SimpleNet(nn.Module):
    def __init__(self):
        super().__init__()
        self.fc = nn.Linear(28*28, 10)
    def forward(self, x):
        x = x.view(x.size(0), -1)
        return self.fc(x)

# Load the trained model weights (simulate loading)
model = SimpleNet()
# Normally you would load weights here, e.g., model.load_state_dict(torch.load('model.pt'))
model.eval()

# Create dummy input with batch size 1 and image size 28x28
dummy_input = torch.randn(1, 1, 28, 28)

# Export the model to ONNX format
onnx_path = "simple_net.onnx"
torch.onnx.export(
    model,
    dummy_input,
    onnx_path,
    export_params=True,
    opset_version=11,
    do_constant_folding=True,
    input_names=["input"],
    output_names=["output"],
    dynamic_axes={"input": {0: "batch_size"}, "output": {0: "batch_size"}}
)

print(f"Model exported to {onnx_path}")

Added torch.onnx.export call to save the PyTorch model in ONNX format.

Used a dummy input tensor with correct shape (1, 1, 28, 28) for export.

Specified input and output node names for clarity.

Set opset_version to 11 for compatibility.

Enabled dynamic axes for batch size to allow variable input sizes.

Results Interpretation

Before: Model saved only in PyTorch format (.pt), limiting interoperability.

After: Model exported to ONNX format with input/output names and dynamic batch size support, enabling use in other frameworks.

Exporting a PyTorch model to ONNX format allows you to use the model in different environments and frameworks, improving flexibility and deployment options.

Bonus Experiment

Load the exported ONNX model using onnxruntime and run inference on a sample input to verify correctness.

💡 Hint

Use onnxruntime.InferenceSession to load the model and run session.run with the input dictionary.

Practice

1. What is the main purpose of exporting a PyTorch model to ONNX format?

easy

A. To save the model in a universal format for sharing and deployment

B. To train the model faster on GPUs

C. To convert the model into a TensorFlow model automatically

D. To visualize the model architecture in PyTorch

Solution

1. Step 1: Understand ONNX export purpose

   ONNX is designed to save models in a format that can be used across different frameworks and platforms.

2. Step 2: Compare options

   Only To save the model in a universal format for sharing and deployment correctly describes the universal sharing and deployment purpose of ONNX export.

3. Final Answer:

   To save the model in a universal format for sharing and deployment -> Option A

4. Quick Check:

   ONNX export = universal format [OK]

Hint: ONNX = share and deploy models universally [OK]

Common Mistakes:

• Confusing ONNX export with training speedup
• Thinking ONNX converts models to TensorFlow automatically
• Assuming ONNX export is for visualization only

2. Which of the following is the correct way to prepare a PyTorch model for ONNX export?

easy

A. Call model.train() before export

B. Export without setting any input

C. Use a dummy input tensor matching the model input shape

D. Use model.eval() after export

Solution

1. Step 1: Identify preparation steps for ONNX export

   Model should be in evaluation mode and a dummy input tensor matching input shape is needed for tracing.

2. Step 2: Evaluate options

   Use a dummy input tensor matching the model input shape correctly states the use of a dummy input tensor. Call model.train() before export is wrong because model.train() is for training mode. Export without setting any input misses input, and Use model.eval() after export is incorrect order.

3. Final Answer:

   Use a dummy input tensor matching the model input shape -> Option C

4. Quick Check:

   Dummy input needed = Use a dummy input tensor matching the model input shape [OK]

Hint: Always use dummy input tensor before export [OK]

Common Mistakes:

• Forgetting to set model.eval() before export
• Not providing dummy input tensor
• Calling model.train() instead of eval()

3. Given the code below, what will be the output of print(onnx_model.graph.input[0].name) after export?

import torch
import torch.nn as nn

class SimpleModel(nn.Module):
    def forward(self, x):
        return x * 2

model = SimpleModel()
model.eval()
dummy_input = torch.randn(1, 3)
torch.onnx.export(model, dummy_input, "model.onnx", input_names=["input_tensor"])
import onnx
onnx_model = onnx.load("model.onnx")
print(onnx_model.graph.input[0].name)

medium

A. x

B. input_tensor

C. input0

D. data

Solution

1. Step 1: Check input_names parameter in export

   The export call sets input_names=["input_tensor"], so the input name in ONNX graph should be "input_tensor".

2. Step 2: Confirm printed input name

   Loading the ONNX model and printing the first input name will output "input_tensor" as set.

3. Final Answer:

   input_tensor -> Option B

4. Quick Check:

   input_names param = input_tensor [OK]

Hint: input_names param sets ONNX input name [OK]

Common Mistakes:

• Assuming default input name like 'input0'
• Confusing PyTorch variable name with ONNX input name
• Not setting input_names and expecting custom name

4. You try to export a PyTorch model to ONNX but get an error: RuntimeError: Input type (torch.cuda.FloatTensor) and weight type (torch.FloatTensor) should be the same. What is the best fix?

medium

A. Export without specifying input_names

B. Use model.train() instead of model.eval()

C. Remove dummy input tensor

D. Move both the model and the dummy input to CPU before export

Solution

1. Step 1: Understand the error cause

   The error means model weights are on CPU but input tensor is on GPU (cuda), causing type mismatch.

2. Step 2: Fix by aligning device

   Moving both the model and the dummy input to CPU ensures both are on the same device (CPU), fixing the mismatch.

3. Final Answer:

   Move both the model and the dummy input to CPU before export -> Option D

4. Quick Check:

   Device mismatch fix = move model and input to CPU [OK]

Hint: Ensure model and input are on same device before export [OK]

Common Mistakes:

• Ignoring device mismatch and exporting anyway
• Switching to train mode instead of fixing device
• Removing dummy input tensor causing other errors

5. You want to export a PyTorch model to ONNX with dynamic batch size support. Which argument should you add to torch.onnx.export to enable this?

hard

A. dynamic_axes={'input': {0: 'batch_size'}, 'output': {0: 'batch_size'}}

B. enable_dynamic_batch=True

C. set_dynamic=True

D. dynamic_batch=True

Solution

1. Step 1: Identify how to specify dynamic axes in ONNX export

   PyTorch ONNX export uses the dynamic_axes argument to mark dimensions as dynamic, e.g., batch size dimension 0.

2. Step 2: Check options for dynamic batch size

   Only dynamic_axes={'input': {0: 'batch_size'}, 'output': {0: 'batch_size'}} correctly uses dynamic_axes with dictionary specifying batch dimension 0 for input and output.

3. Final Answer:

   dynamic_axes={'input': {0: 'batch_size'}, 'output': {0: 'batch_size'}} -> Option A

4. Quick Check:

   Dynamic batch size = dynamic_axes param [OK]

Hint: Use dynamic_axes dict to set dynamic batch size [OK]

Common Mistakes:

• Using nonexistent parameters like enable_dynamic_batch
• Forgetting to specify dynamic axes for output
• Assuming batch size is dynamic by default