In your __init__ method you are not assigning models.vgg16() to a class attribute, but to a local variable vgg16, which would thus not be available in the forward method.
Use self.vgg16 = models.vgg16(pretrained=True) and self.vgg16 in all following operations and it should work.
PS: you can post code snippets by wrapping them into three backticks ```, which makes debugging easier.