I have trained a model using the following code in test_custom_resnet18.ipynb.
from __future__ import print_function, division

import torch
import torch.nn as nn
import torch.optim as optim
from torch.optim import lr_scheduler
import numpy as np
import torchvision
from torchvision import datasets, models, transforms
import matplotlib.pyplot as plt
import time
import os
import copy

plt.ion()

# Data augmentation and normalization for training
# Just normalization for validation
data_transforms = {
    'train': transforms.Compose([
        transforms.RandomResizedCrop(224),
        transforms.RandomHorizontalFlip(),
        transforms.ToTensor(),
        transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
    ]),
    'val': transforms.Compose([
        transforms.Resize(256),
        transforms.CenterCrop(224),
        transforms.ToTensor(),
        transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
    ]),
}

data_dir = 'hymenoptera_data'
image_datasets = {x: datasets.ImageFolder(os.path.join(data_dir, x),
                                          data_transforms[x])
                  for x in ['train', 'val']}
dataloaders = {x: torch.utils.data.DataLoader(image_datasets[x], batch_size=4,
                                             shuffle=True, num_workers=4)
              for x in ['train', 'val']}
dataset_sizes = {x: len(image_datasets[x]) for x in ['train', 'val']}
class_names = image_datasets['train'].classes

device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")

def imshow(inp, title=None):
    """Imshow for Tensor."""
    inp = inp.numpy().transpose((1, 2, 0))
    mean = np.array([0.485, 0.456, 0.406])
    std = np.array([0.229, 0.224, 0.225])
    inp = std * inp + mean
    inp = np.clip(inp, 0, 1)
    plt.imshow(inp)
    if title is not None:
        plt.title(title)
    plt.pause(0.001)  # pause a bit so that plots are updated


# Get a batch of training data
inputs, classes = next(iter(dataloaders['train']))

# Make a grid from batch
out = torchvision.utils.make_grid(inputs)

imshow(out, title=[class_names[x] for x in classes])

def train_model(model, criterion, optimizer, scheduler, num_epochs=25):
    since = time.time()

    best_model_wts = copy.deepcopy(model.state_dict())
    best_acc = 0.0

    for epoch in range(num_epochs):
        print('Epoch {}/{}'.format(epoch, num_epochs - 1))
        print('-' * 10)

        # Each epoch has a training and validation phase
        for phase in ['train', 'val']:
            if phase == 'train':
                model.train()  # Set model to training mode
            else:
                model.eval()   # Set model to evaluate mode

            running_loss = 0.0
            running_corrects = 0

            # Iterate over data.
            for inputs, labels in dataloaders[phase]:
                inputs = inputs.to(device)
                labels = labels.to(device)

                # zero the parameter gradients
                optimizer.zero_grad()

                # forward
                # track history if only in train
                with torch.set_grad_enabled(phase == 'train'):
                    outputs = model(inputs)
                    _, preds = torch.max(outputs, 1)
                    loss = criterion(outputs, labels)

                    # backward + optimize only if in training phase
                    if phase == 'train':
                        loss.backward()
                        optimizer.step()

                # statistics
                running_loss += loss.item() * inputs.size(0)
                running_corrects += torch.sum(preds == labels.data)
            if phase == 'train':
                scheduler.step()

            epoch_loss = running_loss / dataset_sizes[phase]
            epoch_acc = running_corrects.double() / dataset_sizes[phase]

            print('{} Loss: {:.4f} Acc: {:.4f}'.format(
                phase, epoch_loss, epoch_acc))

            # deep copy the model
            if phase == 'val' and epoch_acc > best_acc:
                best_acc = epoch_acc
                best_model_wts = copy.deepcopy(model.state_dict())

        print()

    time_elapsed = time.time() - since
    print('Training complete in {:.0f}m {:.0f}s'.format(
        time_elapsed // 60, time_elapsed % 60))
    print('Best val Acc: {:4f}'.format(best_acc))

    # load best model weights
    model.load_state_dict(best_model_wts)
    return model

# using the custom resnet18 
import custom_resnet18
model_ft = custom_resnet18.ResNet18()
num_ftrs = model_ft.fc.in_features
# Here the size of each output sample is set to 2.
# Alternatively, it can be generalized to nn.Linear(num_ftrs, len(class_names)).
model_ft.fc = nn.Linear(num_ftrs, 2)

model_ft = model_ft.to(device)

criterion = nn.CrossEntropyLoss()

# Observe that all parameters are being optimized
optimizer_ft = optim.SGD(model_ft.parameters(), lr=0.001, momentum=0.9)

# Decay LR by a factor of 0.1 every 7 epochs
exp_lr_scheduler = lr_scheduler.StepLR(optimizer_ft, step_size=7, gamma=0.1)

model_ft = train_model(model_ft, criterion, optimizer_ft,
                         exp_lr_scheduler, num_epochs=25)

wherein, custom_resnet18.py is:
import torch
import torch.nn as nn

class Block(nn.Module):
    def __init__(self, num_layers, in_channels, out_channels, identity_downsample=None, stride=1):
        assert num_layers in [18, 34, 50, 101, 152], "should be a a valid architecture"
        super(Block, self).__init__()
        self.num_layers = num_layers
        if self.num_layers > 34:
            self.expansion = 4
        else:
            self.expansion = 1
        # ResNet50, 101, and 152 include additional layer of 1x1 kernels
        self.conv1 = nn.Conv2d(in_channels, out_channels, kernel_size=1, stride=1, padding=0)
        self.bn1 = nn.BatchNorm2d(out_channels)
        if self.num_layers > 34:
            self.conv2 = nn.Conv2d(out_channels, out_channels, kernel_size=3, stride=stride, padding=1)
        else:
            # for ResNet18 and 34, connect input directly to (3x3) kernel (skip first (1x1))
            self.conv2 = nn.Conv2d(in_channels, out_channels, kernel_size=3, stride=stride, padding=1)
        self.bn2 = nn.BatchNorm2d(out_channels)
        self.conv3 = nn.Conv2d(out_channels, out_channels * self.expansion, kernel_size=1, stride=1, padding=0)
        self.bn3 = nn.BatchNorm2d(out_channels * self.expansion)
        self.relu = nn.ReLU()
        self.identity_downsample = identity_downsample

    def forward(self, x):
        identity = x
        if self.num_layers > 34:
            x = self.conv1(x)
            x = self.bn1(x)
            x = self.relu(x)
        x = self.conv2(x)
        x = self.bn2(x)
        x = self.relu(x)
        x = self.conv3(x)
        x = self.bn3(x)

        if self.identity_downsample is not None:
            identity = self.identity_downsample(identity)

        x += identity
        x = self.relu(x)
        return x


class ResNet(nn.Module):
    def __init__(self, num_layers, block, image_channels, num_classes):
        assert num_layers in [18, 34, 50, 101, 152], f'ResNet{num_layers}: Unknown architecture! Number of layers has ' \
                                                     f'to be 18, 34, 50, 101, or 152 '
        super(ResNet, self).__init__()
        if num_layers < 50:
            self.expansion = 1
        else:
            self.expansion = 4
        if num_layers == 18:
            layers = [2, 2, 2, 2]
        elif num_layers == 34 or num_layers == 50:
            layers = [3, 4, 6, 3]
        elif num_layers == 101:
            layers = [3, 4, 23, 3]
        else:
            layers = [3, 8, 36, 3]
        self.in_channels = 64
        self.conv1 = nn.Conv2d(image_channels, 64, kernel_size=7, stride=2, padding=3)
        self.bn1 = nn.BatchNorm2d(64)
        self.relu = nn.ReLU()
        self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)

        # ResNetLayers
        self.layer1 = self.make_layers(num_layers, block, layers[0], intermediate_channels=64, stride=1)
        self.layer2 = self.make_layers(num_layers, block, layers[1], intermediate_channels=128, stride=2)
        self.layer3 = self.make_layers(num_layers, block, layers[2], intermediate_channels=256, stride=2)
        self.layer4 = self.make_layers(num_layers, block, layers[3], intermediate_channels=512, stride=2)

        self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
        self.fc = nn.Linear(512 * self.expansion, num_classes)

    def forward(self, x):
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)

        x = self.layer1(x)
        x = self.layer2(x)
        x = self.layer3(x)
        x = self.layer4(x)

        x = self.avgpool(x)
        x = x.reshape(x.shape[0], -1)
        x = self.fc(x)
        return x

    def make_layers(self, num_layers, block, num_residual_blocks, intermediate_channels, stride):
        layers = []

        identity_downsample = nn.Sequential(nn.Conv2d(self.in_channels, intermediate_channels*self.expansion, kernel_size=1, stride=stride),
                                            nn.BatchNorm2d(intermediate_channels*self.expansion))
        layers.append(block(num_layers, self.in_channels, intermediate_channels, identity_downsample, stride))
        self.in_channels = intermediate_channels * self.expansion # 256
        for i in range(num_residual_blocks - 1):
            layers.append(block(num_layers, self.in_channels, intermediate_channels)) # 256 -> 64, 64*4 (256) again
        return nn.Sequential(*layers)


def ResNet18(img_channels=3, num_classes=1000):
    return ResNet(18, Block, img_channels, num_classes)


def ResNet34(img_channels=3, num_classes=1000):
    return ResNet(34, Block, img_channels, num_classes)


def ResNet50(img_channels=3, num_classes=1000):
    return ResNet(50, Block, img_channels, num_classes)


def ResNet101(img_channels=3, num_classes=1000):
    return ResNet(101, Block, img_channels, num_classes)


def ResNet152(img_channels=3, num_classes=1000):
    return ResNet(152, Block, img_channels, num_classes)


def test():
    net = ResNet18(img_channels=3, num_classes=1000)
    y = net(torch.randn(4, 3, 224, 224)).to("cuda")
    print(y.size())
    
test()

I want to extract the feature vector right before the fully-connected (FC) layer. When I use the following code, it only prints the model not the actual value of feature vector. How can I access the actual value of the feature vector?
nn.Sequential(*list(model_ft.children())[:-1])

It prints:
Sequential(
  (0): Conv2d(3, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3))
  (1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
  (2): ReLU()
  (3): MaxPool2d(kernel_size=3, stride=2, padding=1, dilation=1, ceil_mode=False)
  (4): Sequential(
    (0): Block(
      (conv1): Conv2d(64, 64, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (bn2): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(64, 64, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
      (identity_downsample): Sequential(
        (0): Conv2d(64, 64, kernel_size=(1, 1), stride=(1, 1))
        (1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      )
    )
    (1): Block(
      (conv1): Conv2d(64, 64, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (bn2): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(64, 64, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
    )
  )
  (5): Sequential(
    (0): Block(
      (conv1): Conv2d(64, 128, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(64, 128, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1))
      (bn2): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(128, 128, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
      (identity_downsample): Sequential(
        (0): Conv2d(64, 128, kernel_size=(1, 1), stride=(2, 2))
        (1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      )
    )
    (1): Block(
      (conv1): Conv2d(128, 128, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (bn2): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(128, 128, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
    )
  )
  (6): Sequential(
    (0): Block(
      (conv1): Conv2d(128, 256, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(128, 256, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1))
      (bn2): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(256, 256, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
      (identity_downsample): Sequential(
        (0): Conv2d(128, 256, kernel_size=(1, 1), stride=(2, 2))
        (1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      )
    )
    (1): Block(
      (conv1): Conv2d(256, 256, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (bn2): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(256, 256, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(256, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
    )
  )
  (7): Sequential(
    (0): Block(
      (conv1): Conv2d(256, 512, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(256, 512, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1))
      (bn2): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(512, 512, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
      (identity_downsample): Sequential(
        (0): Conv2d(256, 512, kernel_size=(1, 1), stride=(2, 2))
        (1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      )
    )
    (1): Block(
      (conv1): Conv2d(512, 512, kernel_size=(1, 1), stride=(1, 1))
      (bn1): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv2): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (bn2): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (conv3): Conv2d(512, 512, kernel_size=(1, 1), stride=(1, 1))
      (bn3): BatchNorm2d(512, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
      (relu): ReLU()
    )
  )
  (8): AdaptiveAvgPool2d(output_size=(1, 1))
)

I also have:
children_counter = 0
for n,c in model_ft.named_children():
    print("Children Counter: ",children_counter," Layer Name: ",n,)
    children_counter+=1

Output:
Children Counter:  0  Layer Name:  conv1
Children Counter:  1  Layer Name:  bn1
Children Counter:  2  Layer Name:  relu
Children Counter:  3  Layer Name:  maxpool
Children Counter:  4  Layer Name:  layer1
Children Counter:  5  Layer Name:  layer2
Children Counter:  6  Layer Name:  layer3
Children Counter:  7  Layer Name:  layer4
Children Counter:  8  Layer Name:  avgpool
Children Counter:  9  Layer Name:  fc

Codes are from Transfer Learning for Computer Vision Tutorial — PyTorch Tutorials 1.9.0+cu102 documentation and ResNet.ipynb · GitHub