RagTokenForGeneration: Fixed parameter name for logits_processor
# What does this PR do?
The parameter name for the beam_search and greedy_search functions of the GenerationMixin is (now?) logits_processor not pre_processor.  This fix makes prefix_allowed_tokens_fn work (again?).

## Who can review?

 Rag: @patrickvonplaten, @lhoestq
 
