How to train a Model which can check whether image is from existing classes or not I have given with a image dataset of 1000 classes , each class has 100 images. Now My requirement is to train a model which will take a image as input, and it should answer whether the image is present or not, there is no need to specify which class it is.. It just has to say yes or no. 

I thought of using a CNN with a sigmoid function at last layer, but not able to connect the dots.
There is an example of classification in this repository:
DogVsCat