Fixes bug in the creation of ExponentialDecayLengthPenalty (#21423)

input_ids_seq_length doesn't exist in the GenerationConfig, it exists as local variable in the function.

Setting exponential_decay_length_penalty therefore results in an error:
`AttributeError: 'GenerationConfig' object has no attribute 'input_ids_seq_length'`

This simple change fixes this issue, and the exponential_decay_length_penalty works as expected.

src/transformers/generation/utils.py

@@ -859,7 +859,7 @@ class GenerationMixin:
                 ExponentialDecayLengthPenalty(
                     generation_config.exponential_decay_length_penalty,
                     generation_config.eos_token_id,
-                    generation_config.input_ids_seq_length,
+                    input_ids_seq_length,
                 )
             )
         if generation_config.suppress_tokens is not None: