Repository: transformers
Issue: NSP Support for Zero-shot Text Classification Pipeline
Labels: Core: Pipeline,Feature request

### Feature request

Zero-shot classification can be solved with NextSentencePrediction task of BERT, and it has shown competitive results to NLI-based zero-shot classification in some cases. There could be a parameter where we choose the type of submethod that we are going to use for the pipeline like `pipeline(task="zero-shot-classification", type_="nsp")` or we could just simply add a task named "nsp-zeroshot-classification". This is also possible for MLM, which is a more widely used pretraining task across LMs.

### Motivation

Like I said, NSP has proven to be useful especially for languages that do not have access to NLI dataset since only pre-training is enough. Although multilingual NLI models can also be used, they have been proven to be worse compared to smaller monolingual models in this task, as one would expect. Even if this is a small detail which would be unnecessary to put into the codebase, I wanted to share this implementation so that anyone who's interested can take a look and try different methods.

Here are some references, one of which is my study, that use NSP for zero-shot classification.

Sun, Y., Zheng, Y., Hao, C., & Qiu, H. (2021). NSP-BERT: A Prompt-based Zero-Shot Learner Through an Original Pre-training Task--Next Sentence Prediction. arXiv preprint arXiv:2109.03564.

Çelik, E., & Dalyan, T. (2023). Unified benchmark for zero-shot Turkish text classification. Information Processing & Management, 60(3), 103298.

### Your contribution

I can open a PR, here's the implementation I did based on Sun et al. 2021. It is heaily based on the current NLI zeroshot pipeline class, but also adds a `reverse` argument which changes the order of the sentences for NSP.

```python
import numpy as np
from typing import List, Union

from transformers.utils import logging
from transformers.pipelines.base import ChunkPipeline, ArgumentHandler
from transformers.tokenization_utils import TruncationStrategy
from transformers.pipelines import ZeroShotClassificationArgumentHandler

logger = logging.get_logger(__name__)


class ZeroShotClassificationArgumentHandler(ArgumentHandler):
    def _parse_labels(self, labels):
        if isinstance(labels, str):
            labels = [label.strip() for label in labels.split(",") if label.strip()]
        return labels

    def __call__(self, sequences, labels, hypothesis_template, reverse):
        if len(labels) == 0 or len(sequences) == 0:
            raise ValueError(
                "You must include at least one label and at least one sequence."
            )
        if hypothesis_template.format(labels[0]) == hypothesis_template:
            raise ValueError(
                (
                    'The provided hypothesis_template "{}" was not able to be formatted with the target labels. '
                    "Make sure the passed template includes formatting syntax such as {{}} where the label should go."
                ).format(hypothesis_template)
            )

        if isinstance(sequences, str):
            sequences = [sequences]

        sequence_pairs = []
        for sequence in sequences:
            if reverse:
                sequence_pairs.extend(
                    [[hypothesis_template.format(label), sequence] for label in labels]
                )
            else:
                sequence_pairs.extend(
                    [[sequence, hypothesis_template.format(label)] for label in labels]
                )

        return sequence_pairs, sequences


class NSPZeroShotClassificationPipeline(ChunkPipeline):
    def __init__(
        self, args_parser=ZeroShotClassificationArgumentHandler(), *args, **kwargs
    ):
        self._args_parser = args_parser
        super().__init__(*args, **kwargs)

    @property
    def isNext_id(self):
        return 0

    def _parse_and_tokenize(
        self,
        sequence_pairs,
        padding=True,
        add_special_tokens=True,
        truncation=TruncationStrategy.ONLY_FIRST,
        **kwargs,
    ):
        return_tensors = self.framework
        if self.tokenizer.pad_token is None:
            logger.error(
                "Tokenizer was not supporting padding necessary for zero-shot, attempting to use "
                " `pad_token=eos_token`"
            )
            self.tokenizer.pad_token = self.tokenizer.eos_token
        try:
            inputs = self.tokenizer(
                sequence_pairs,
                add_special_tokens=add_special_tokens,
                return_tensors=return_tensors,
                padding=padding,
                truncation=truncation,
            )
        except Exception as e:
            if "too short" in str(e):
                inputs = self.tokenizer(
                    sequence_pairs,
                    add_special_tokens=add_special_tokens,
                    return_tensors=return_tensors,
                    padding=padding,
                    truncation=TruncationStrategy.DO_NOT_TRUNCATE,
                )
            else:
                raise e

        return inputs

    def _sanitize_parameters(self, **kwargs):
        if kwargs.get("multi_class", None) is not None:
            kwargs["multi_label"] = kwargs["multi_class"]
            logger.warning(
                "The `multi_class` argument has been deprecated and renamed to `multi_label`. "
                "`multi_class` will be removed in a future version of Transformers."
            )
        preprocess_params = {}
        if "candidate_labels" in kwargs:
            preprocess_params["candidate_labels"] = self._args_parser._parse_labels(
                kwargs["candidate_labels"]
            )
        if "hypothesis_template" in kwargs:
            preprocess_params["hypothesis_template"] = kwargs["hypothesis_template"]
        if "reverse" in kwargs:
            preprocess_params["reverse"] = kwargs["reverse"]

        postprocess_params = {}
        if "multi_label" in kwargs:
            postprocess_params["multi_label"] = kwargs["multi_label"]
        return preprocess_params, {}, postprocess_params

    def __call__(
        self,
        sequences: Union[str, List[str]],
        *args,
        **kwargs,
    ):
        if len(args) == 0:
            pass
        elif len(args) == 1 and "candidate_labels" not in kwargs:
            kwargs["candidate_labels"] = args[0]
        else:
            raise ValueError(f"Unable to understand extra arguments {args}")

        return super().__call__(sequences, **kwargs)

    def preprocess(
        self,
        inputs,
        candidate_labels=None,
        hypothesis_template="This example is {}.",
        reverse=False,
    ):
        sequence_pairs, sequences = self._args_parser(
            inputs, candidate_labels, hypothesis_template, reverse
        )

        for i, (candidate_label, sequence_pair) in enumerate(
            zip(candidate_labels, sequence_pairs)
        ):
            model_input = self._parse_and_tokenize([sequence_pair])

            yield {
                "candidate_label": candidate_label,
                "sequence": sequences[0],
                "is_last": i == len(candidate_labels) - 1,
                **model_input,
            }

    def _forward(self, inputs):
        candidate_label = inputs["candidate_label"]
        sequence = inputs["sequence"]
        model_inputs = {k: inputs[k] for k in self.tokenizer.model_input_names}
        outputs = self.model(**model_inputs)

        model_outputs = {
            "candidate_label": candidate_label,
            "sequence": sequence,
            "is_last": inputs["is_last"],
            **outputs,
        }
        return model_outputs

    def postprocess(self, model_outputs, multi_label=False):
        candidate_labels = [outputs["candidate_label"] for outputs in model_outputs]
        sequences = [outputs["sequence"] for outputs in model_outputs]
        logits = np.concatenate([output["logits"].numpy() for output in model_outputs])
        N = logits.shape[0]
        n = len(candidate_labels)
        num_sequences = N // n
        reshaped_outputs = logits.reshape((num_sequences, n, -1))

        if multi_label or len(candidate_labels) == 1:
            isNext_id = self.isNext_id
            notNext_id = 1
            isNext_contr_logits = reshaped_outputs[..., [notNext_id, isNext_id]]
            scores = np.exp(isNext_contr_logits) / np.exp(isNext_contr_logits).sum(
                -1, keepdims=True
            )
            scores = scores[..., 1]
        else:
            isNext_logits = reshaped_outputs[..., self.isNext_id]
            scores = np.exp(isNext_logits) / np.exp(isNext_logits).sum(
                -1, keepdims=True
            )

        top_inds = list(reversed(scores[0].argsort()))
        return {
            "sequence": sequences[0],
            "labels": [candidate_labels[i] for i in top_inds],
            "scores": scores[0, top_inds].tolist(),
        }

```

This task can be used by registering it to the tasks, shown in example below:
```python
from nsp import NSPZeroShotClassificationPipeline
from transformers.pipelines import PIPELINE_REGISTRY
from transformers import BertForNextSentencePrediction, TFBertForNextSentencePrediction

PIPELINES = [
    dict(
        task="nsp-zeroshot-classification",
        pipeline_class=NSPZeroShotClassificationPipeline,
        pt_model=BertForNextSentencePrediction,
        tf_model=TFBertForNextSentencePrediction,
        default={"pt": ("bert-base-uncased")},
        type="text",
    )
]

for p in PIPELINES:
    PIPELINE_REGISTRY.register_pipeline(**p)
```