Fix contrastive search to correctly handle input with padding (#33507)
* fix: handle padding in contrastive search for decoder-only models
* fix: handle padding in contrastive search for encoder-decoder models
* tests: move padding contrastive test to test_util, add t5 test
* fix: handle if model_kwargs["decoder_attention_mask"] is None
* refactor: improve padding input contrastive search generation tests
* chore: _ranking_fast to use LongTensor for cosine_matrix_mask