Fix task pre-processor in tasks

[sampathweb]

When a user passes preprocessor to a from_preset constructor via kwargs, it fails with "got multiple values for keyword argument 'preprocessor'". This is because from_preset adds preprocessor parameter regardless of whether has also provided it. With this fix, we only add the preprocessor when the user has not provided the input.

Also, adds a validation to verify that user doesn't provide a backbone to the from_preset method since that's obtained from the preset directly.

# Repro steps
preprocessor = keras_nlp.models.DistilBertPreprocessor.from_preset(
    "distil_bert_base_en_uncased",
    sequence_length=SEQ_LENGTH,
)

# Pretrained classifier -  Errors out with duplicate `preprocessor` arg
classifier = keras_nlp.models.DistilBertClassifier.from_preset(
    "distil_bert_base_en_uncased",
    num_classes=2,
    activation=None,
    preprocessor=preprocessor,
)

[mattdangerw]

Thank you! Couple comments.

[mattdangerw]

lgtm!

[nkovela1]

LGTM, thank you for the fix Ramesh!