Print correct number of key value heads on dimension assertion.

[dstripelis]

What does this PR do?

Fixes # (issue)
Change config.num_key_value_heads to num_key_value_heads print throughout under lorax_server/models/custom_modeling.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue or the discord / slack channel? Please add a link
  to it if that's the case.
• Did you write any new necessary tests?

Who can review?

@tgaddair

[tgaddair]

Thanks! Good catch.