Fixes bug in the creation of ExponentialDecayLengthPenalty
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.

@gante 
_The documentation is not available anymore as the PR was closed or merged._<|||||>merging as the failing test (`tests/models/cvt/test_modeling_cvt.py::CvtModelTest::test_save_load_fast_init_to_base`) is a known flaky test