There are a few things you want to resolve here:
Maintaining User Orientation
You don't want to disorient the user by forcing a log out.
I'd opt for a confirmation screen, and an email confirmation sent. Perhaps with a message indicating that if they did not request this action, provide a method of Undo or Reset again.
Also, if they are locked out, they may not be able to get back in again to change it to something more memorable if they forget their password right away.
Throwing them into the "logged out" screen right away seems like a great idea in theory as it helps the user reinforce their memory, but can be jarring and give the impression that something went wrong.
re: Short Term Memory Load
Some users have short term memory loss due to:
- The use of Password strength indicators (They may have gone over too many)
- Memorization without visual representation (Bullets are difficult to memorize)
- Password character requirements (Pushes users out of their mnemonic comfort zone to create something unreadable)