code: # Copyright (c) Microsoft Corporation.
  # Licensed under the MIT license.
  
  '''
  NNI example for supported ActivationAPoZRank and ActivationMeanRank pruning algorithms.
  In this example, we show the end-to-end pruning process: pre-training -> pruning -> fine-tuning.
  Note that pruners use masks to simulate the real pruning. In order to obtain a real compressed model, model speed up is required.
  
  '''
  import argparse
  import sys
  
  import torch
  from torchvision import datasets, transforms
  from torch.optim.lr_scheduler import MultiStepLR
  
  from nni.compression.pytorch import ModelSpeedup
  from nni.compression.pytorch.utils.counter import count_flops_params
  from nni.algorithms.compression.v2.pytorch.pruning.basic_pruner import ActivationAPoZRankPruner, ActivationMeanRankPruner
  from nni.algorithms.compression.v2.pytorch.utils import trace_parameters
  
  from pathlib import Path
  sys.path.append(str(Path(__file__).absolute().parents[2] / 'models'))
  from cifar10.vgg import VGG
  
  device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
  normalize = transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2023, 0.1994, 0.2010))
  g_epoch = 0
  
  train_loader = torch.utils.data.DataLoader(
      datasets.CIFAR10('./data', train=True, transform=transforms.Compose([
          transforms.RandomHorizontalFlip(),
          transforms.RandomCrop(32, 4),
          transforms.ToTensor(),
          normalize,
      ]), download=True),
      batch_size=128, shuffle=True)
  
  test_loader = torch.utils.data.DataLoader(
      datasets.CIFAR10('./data', train=False, transform=transforms.Compose([
          transforms.ToTensor(),
          normalize,
      ])),
      batch_size=128, shuffle=False)
  
  def trainer(model, optimizer, criterion):
      global g_epoch
      model.train()
      for batch_idx, (data, target) in enumerate(train_loader):
          data, target = data.to(device), target.to(device)
          optimizer.zero_grad()
          output = model(data)
          loss = criterion(output, target)
          loss.backward()
          optimizer.step()
          if batch_idx and batch_idx % 100 == 0:
              print('Train Epoch: {} [{}/{} ({:.0f}%)]\tLoss: {:.6f}'.format(
                  g_epoch, batch_idx * len(data), len(train_loader.dataset),
                  100. * batch_idx / len(train_loader), loss.item()))
      g_epoch += 1
  
  def evaluator(model):
      model.eval()
      correct = 0.0
      with torch.no_grad():
          for data, target in test_loader:
              data, target = data.to(device), target.to(device)
              output = model(data)
              pred = output.argmax(dim=1, keepdim=True)
              correct += pred.eq(target.view_as(pred)).sum().item()
      acc = 100 * correct / len(test_loader.dataset)
      print('Accuracy: {}%\n'.format(acc))
      return acc
  
  def optimizer_scheduler_generator(model, _lr=0.1, _momentum=0.9, _weight_decay=5e-4, total_epoch=160):
      optimizer = torch.optim.SGD(model.parameters(), lr=_lr, momentum=_momentum, weight_decay=_weight_decay)
      scheduler = MultiStepLR(optimizer, milestones=[int(total_epoch * 0.5), int(total_epoch * 0.75)], gamma=0.1)
      return optimizer, scheduler
  
  if __name__ == '__main__':
      parser = argparse.ArgumentParser(description='PyTorch Example for model comporession')
      parser.add_argument('--pruner', type=str, default='apoz',
                          choices=['apoz', 'mean'],
                          help='pruner to use')
      parser.add_argument('--pretrain-epochs', type=int, default=20,
                          help='number of epochs to pretrain the model')
      parser.add_argument('--fine-tune-epochs', type=int, default=20,
                          help='number of epochs to fine tune the model')
      args = parser.parse_args()
  
      print('\n' + '=' * 50 + ' START TO TRAIN THE MODEL ' + '=' * 50)
      model = VGG().to(device)
      optimizer, scheduler = optimizer_scheduler_generator(model, total_epoch=args.pretrain_epochs)
      criterion = torch.nn.CrossEntropyLoss()
      pre_best_acc = 0.0
      best_state_dict = None
  
      for i in range(args.pretrain_epochs):
          trainer(model, optimizer, criterion)
          scheduler.step()
          acc = evaluator(model)
          if acc > pre_best_acc:
              pre_best_acc = acc
              best_state_dict = model.state_dict()
      print("Best accuracy: {}".format(pre_best_acc))
      model.load_state_dict(best_state_dict)
      pre_flops, pre_params, _ = count_flops_params(model, torch.randn([128, 3, 32, 32]).to(device))
      g_epoch = 0
  
      # Start to prune and speedup
      print('\n' + '=' * 50 + ' START TO PRUNE THE BEST ACCURACY PRETRAINED MODEL ' + '=' * 50)
      config_list = [{
          'total_sparsity': 0.5,
          'op_types': ['Conv2d'],
      }]
  
      # make sure you have used nni.algorithms.compression.v2.pytorch.utils.trace_parameters to wrap the optimizer class before initialize
      traced_optimizer = trace_parameters(torch.optim.SGD)(model.parameters(), lr=0.01, momentum=0.9, weight_decay=5e-4)
      if 'apoz' in args.pruner:
          pruner = ActivationAPoZRankPruner(model, config_list, trainer, traced_optimizer, criterion, training_batches=20)
      else:
          pruner = ActivationMeanRankPruner(model, config_list, trainer, traced_optimizer, criterion, training_batches=20)
      _, masks = pruner.compress()
      pruner.show_pruned_weights()
      pruner._unwrap_model()
      ModelSpeedup(model, dummy_input=torch.rand([10, 3, 32, 32]).to(device), masks_file=masks).speedup_model()
      print('\n' + '=' * 50 + ' EVALUATE THE MODEL AFTER SPEEDUP ' + '=' * 50)
      evaluator(model)
  
      # Optimizer used in the pruner might be patched, so recommend to new an optimizer for fine-tuning stage.
      print('\n' + '=' * 50 + ' START TO FINE TUNE THE MODEL ' + '=' * 50)
      optimizer, scheduler = optimizer_scheduler_generator(model, _lr=0.01, total_epoch=args.fine_tune_epochs)
  
      best_acc = 0.0
      g_epoch = 0
      for i in range(args.fine_tune_epochs):
          trainer(model, optimizer, criterion)
          scheduler.step()
          best_acc = max(evaluator(model), best_acc)
      flops, params, results = count_flops_params(model, torch.randn([128, 3, 32, 32]).to(device))
      print(f'Pretrained model FLOPs {pre_flops/1e6:.2f} M, #Params: {pre_params/1e6:.2f}M, Accuracy: {pre_best_acc: .2f}%')
      print(f'Finetuned model FLOPs {flops/1e6:.2f} M, #Params: {params/1e6:.2f}M, Accuracy: {best_acc: .2f}%')
  
apis: torch.randn torch.rand torch.no_grad torch.nn.CrossEntropyLoss torch.cuda.is_available