That's what I did when the same happened to me. Enable iCloud Keychain on the iPad and the Mac, wait for the passwords to sync (might take a few minutes) and then open Keychain Access on the Mac and find the password on it.
This did not work because the Yahoo password is not stored on the device after being entered. Apparently Yahoo has something called XYMCOOKIE authentication for SMTP and IMAP which is basically just acceptance of a pre-sent hash/cookie. The iPad discards the actual password the first time you enter it and instead stores a XYMCOOKIE.
See here: https://www.google.com/search?q=XYMCOOKIE+authentication