Generate: correct beam search length on score calculation for multi batch generation
# What does this PR do?

Fixes #23084 

When computing the score with length penalty, the length was (incorrectly) incremented once per batch member. It should only be incremented once -- the length here is `cur_len` (the length of the generated tokens) + `1` (the token being added at the iteration).

Slow tests were ran for BART, T5, GPT2 -- no regressions.
_The documentation is not available anymore as the PR was closed or merged._