Here you find more details stackoverflow
Usually the dogs and cats examples have only train and valid dataset, where the training dataset is 12500 files per class, and the validation dataset is 1000 files per class (~7% of the data).
What you need to pay attention is when you build your datasets, to cut sample files from training dataset and move then in the validation dataset. (Instead of just copy paste).
If you do kaggle competition for example, you will have also a test dataset (with no labels/classes). In this case, you can train your model using the cross-validation technique. And in the end, you can put all of your files in the training dataset (no more validation set), and this will be your final weights.