Fix backward compatibility for GenerationMode import (#719)

Author: eaidova

optimum/intel/openvino/modeling_decoder.py

@@ -28,10 +28,10 @@
 from transformers import AutoModelForCausalLM, PretrainedConfig
 from transformers.file_utils import add_start_docstrings, add_start_docstrings_to_model_forward
 from transformers.generation import GenerationMixin
-from transformers.generation.configuration_utils import GenerationConfig, GenerationMode
+from transformers.generation.configuration_utils import GenerationConfig
 from transformers.generation.logits_process import LogitsProcessorList
 from transformers.generation.stopping_criteria import StoppingCriteriaList
-from transformers.generation.utils import GenerateOutput
+from transformers.generation.utils import GenerateOutput, GenerationMode
 from transformers.modeling_outputs import CausalLMOutputWithPast
 
 from optimum.utils.normalized_config import NormalizedConfigManager