Compression Evaluator

The Evaluator is used to package the training and evaluation process for a targeted model. To explain why NNI needs an Evaluator, let’s first look at the general process of model compression in NNI.

In model pruning, some algorithms need to prune according to some intermediate variables (gradients, activations, etc.) generated during the training process, and some algorithms need to gradually increase or adjust the sparsity of different layers during the training process, or adjust the pruning strategy according to the performance changes of the model during the pruning process.

In model quantization, NNI has quantization-aware training algorithm, it can adjust the scale and zero point required for model quantization from time to time during the training process, and may achieve a better performance compare to post-training quantization.

In order to better support the above algorithms’ needs and maintain the consistency of the interface, NNI introduces the Evaluator as the carrier of the training and evaluation process.


For users prior to NNI v2.8: NNI previously provided APIs like trainer, traced_optimizer, criterion, finetuner. These APIs were maybe tedious in terms of user experience. Users need to exchange the corresponding API frequently if they want to switch compression algorithms. Evaluator is an alternative to the above interface, users only need to create the evaluator once and it can be used in all compressors.

For users of native PyTorch, TorchEvaluator requires the user to encapsulate the training process as a function and exposes the specified interface, which will bring some complexity. But don’t worry, in most cases, this will not change too much code.

For users of PyTorchLightning, LightningEvaluator can be created with only a few lines of code based on your original Lightning code.

For users of Transformers Trainer, TransformersEvaluator can be created with only a few lines of code.

Here we give three examples of how to create an Evaluator for native PyTorch users, PyTorchLightning users and Huggingface Transformers users.


TorchEvaluator is for the users who work in a native PyTorch environment (If you are using PyTorchLightning, please refer LightningEvaluator).

TorchEvaluator has six initialization parameters training_func, optimizers, training_step, lr_schedulers, dummy_input, evaluating_func.

  • training_func is the training loop to train the compressed model. It is a callable function with six input parameters model, optimizers, training_step, lr_schedulers, max_steps, max_epochs. Please make sure each input argument of the training_func is actually used, especially max_steps and max_epochs can correctly control the duration of training.

  • optimizers is a single / a list of traced optimizer(s), please make sure using nni.trace wrapping the Optimizer class before initializing it / them.

  • training_step A callable function, the first argument of inputs should be batch, and the outputs should contain loss. Three kinds of outputs are supported: single loss, tuple with the first element is loss, a dict contains a key loss.

  • lr_schedulers is a single / a list of traced scheduler(s), same as optimizers, please make sure using nni.trace wrapping the _LRScheduler class before initializing it / them.

  • dummy_input is used to trace the model, same as example_inputs in torch.jit.trace.

  • evaluating_func is a callable function to evaluate the compressed model performance. Its input is a compressed model and its output is metric. The format of metric should be a float number or a dict with key default.

Please refer TorchEvaluator for more details. Here is an example of how to initialize a TorchEvaluator.

def training_step(batch, model, *args, **kwargs):
    output = model(batch[0])
    loss = F.cross_entropy(output, batch[1])
    return loss

def training_func(model, optimizer, training_step, lr_scheduler, max_steps, max_epochs):
    assert max_steps is not None or max_epochs is not None
    total_steps = max_steps if max_steps else max_epochs * len(train_dataloader)
    total_epochs = total_steps // len(train_dataloader) + (0 if total_steps % len(train_dataloader) == 0 else 1)

    current_step = 0
    for _ in range(total_epochs):
        for batch in train_dataloader:
            loss = training_step(batch, model)

            # if reach the total steps, exit from the training loop
            current_step = current_step + 1
            if current_step >= total_steps:

        # if you are using a epoch-wise scheduler, call it here

optimizer = nni.trace(torch.optim.Adam)(model.parameters(), lr=0.001)
lr_scheduler = nni.trace(torch.optim.lr_scheduler.LambdaLR)(optimizer, lr_lambda=lambda epoch: 1 / epoch)

evaluator = TorchEvaluator(training_func, optimizer, training_step, lr_scheduler)


It is also worth to note that not all the arguments of TorchEvaluator must be provided. Some compressors only require evaluate_func as they do not train the model, some compressors only require training_func. Please refer to each compressor’s doc to check the required arguments. But, it is fine to provide more arguments than the compressor’s need.

A complete example can be found here.


LightningEvaluator is for the users who work with PyTorchLightning.

Only three parts users need to modify compared with the original pytorch-lightning code:

  1. Wrap the Optimizer and LRScheduler class with nni.trace.

  2. Wrap the LightningModule class with nni.trace.

  3. Wrap the LightningDataModule class with nni.trace.

Please refer LightningEvaluator for more details. Here is an example of how to initialize a LightningEvaluator.

pl_trainer = nni.trace(pl.Trainer)(...)
pl_data = nni.trace(MyDataModule)(...)

evaluator = LightningEvaluator(pl_trainer, pl_data)


In LightningModule.configure_optimizers, user should use traced torch.optim.Optimizer and traced torch.optim._LRScheduler. It’s for NNI can get the initialization parameters of the optimizers and lr_schedulers.

class SimpleModel(pl.LightningModule):

    def configure_optimizers(self):
        optimizers = nni.trace(torch.optim.SGD)(model.parameters(), lr=0.001)
        lr_schedulers = nni.trace(ExponentialLR)(optimizer=optimizers, gamma=0.1)
        return optimizers, lr_schedulers

A complete example can be found here.


TransformersEvaluator is for the users who work with Huggingface Transformers Trainer.

The only need is using nni.trace to wrap the Trainer class.

import nni
from transformers.trainer import Trainer
trainer = nni.trace(Trainer)(model, training_args, ...)

from nni.contrib.compression.utils import TransformersEvaluator
evaluator = TransformersEvaluator(trainer)

Moreover, if you are utilizing a personalized optimizer or learning rate scheduler, kindly use nni.trace to wrap their class as well.

optimizer = nni.trace(torch.optim.Adam)(model.parameters(), lr=0.001)
lr_scheduler = nni.trace(torch.optim.lr_scheduler.LambdaLR)(optimizer, lr_lambda=lambda epoch: 1 / epoch)