I want to frozen the weight of pretrained vgg19, and just update the last fc layer when backpropagation
The model is shown in following block. I have already set  with torch.no_grad(): when I input the images into vgg19.
class ImgEncoder(nn.Module):

    def __init__(self, embed_dim):

        super(ImgEncoder, self).__init__()
        self.model = models.vgg19(pretrained=True)
        in_features = self.model.classifier[-1].in_features
        self.model.classifier = nn.Sequential(*list(self.model.classifier.children())[:-1]) # remove vgg19 last layer
        self.fc = nn.Linear(in_features, embed_dim)

    def forward(self, image):

        with torch.no_grad():
            img_feature = self.model(image) # (batch, channel, height, width)
        img_feature = self.fc(img_feature)

        l2_norm = F.normalize(img_feature, p=2, dim=1).detach()
        return l2_norm

model = ImgEncoder(embed_dim=300)

the question is when I using the optimizer, how should I feed the parameter
optimizer = optim.Adam(model.parameters(), lr=0.001)

or
optimizer = optim.Adam(model.fc.parameters(), lr=0.001)

This question is similar to the tutorial in official website like following:
image1064×342 28.6 KB
Why we can’t simply put the model.parameters() but model.fc.parameters() instead ?