callback_steps is typed as Optional[int] but has a default value

[williamberman]

In pipelines, we pass both a callback and a callback_steps. callback_steps is always set and has a default value of 1. callback is optional. We always type callback_steps incorrectly as Optional[int] instead of just int. We should update the type signatures of callback_steps.

#2297 (comment)

[patrickvonplaten]

#2357