Give unique IDs to temporary checkpoints.

[chadcwilliams]

checkpoint_01.pt and checkpoint_02.pt are temporary checkpoints for GAN training. However, if multiple GANs are being run simultaneously, they will overwrite the same checkpoint files. As such, we should add a time tag for these checkpoints. Essentially, the idea is to determine the time.time() at the beginning of gan training and append this to the checkpoint filename such as checkpoint_1691507119_01.pt and checkpoint_1691507119_02.pt.

[chadcwilliams]

We decided not to do this.