Add _is_stateful property to OVBaseDecoderModel

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What does this PR do?

Hello, when I try assisted generation with optimum-intel and openvino, it complains about

AttributeError: 'OVModelForCausalLM' object has no attribute '_is_stateful'. Did you mean: '_make_stateful'?

After adding this property to OVBaseDecoderModel, the error is gone, and assisted generation works.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?