Yes, it is Voldemort in the Forbidden Forest that we see in Harry Potter and the Philosopher's Stone when Harry, Hermione and Draco go to detention. However, Voldemort is still not in his human form because of the curse he cast on Harry backfiring. Voldemort was there drinking unicorn's blood because it is said to have rejuvenating properties and the ability to keep something like Voldemort alive until he can find a body.