Thanks for the code.
It looks like you would like to swap the last linear layer of the pretrained ResNet with your nn.Sequential block.
However, resnet does not use self.classifier as its last layer, but self.fc. This also explains the error, since you are currently setting the required_grad flag to False for each layer, while your newly created model.classifier is not being used.
Try to assign your nn.Sequential block to model.fc instead, which should fix the error.