[Add] sequence length to text gen pipelines

[rahul-tuli]

This PR exposes sequence_length as a Pipeline level property

Test Code:

from deepsparse import Pipeline

pipeline = Pipeline.create(
            task="text-generation",
            model_path="zoo:mpt-7b-mpt_pretrain-base_quantized",
        )

assert hasattr(pipeline, "sequence_length")

Was failing before, but passes after this PR