class_docstr: Convert a collection of raw documents to a document id matrix.
  
  Attributes:
      _use_char: boolean. Whether to use char feature.
      _num_norm: boolean. Whether to normalize text.
      _word_vocab: dict. A mapping of words to feature indices.
      _char_vocab: dict. A mapping of chars to feature indices.
      _label_vocab: dict. A mapping of labels to feature indices.
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docstr: Return label strings.
  
  Args:
      y: label id matrix.
      lengths: sentences length.
  
  Returns:
      list: list of list of strings.
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code: def inverse_transform(self, y, lengths=None):
      """Return label strings.
  
      Args:
          y: label id matrix.
          lengths: sentences length.
  
      Returns:
          list: list of list of strings.
      """
      y = np.argmax(y, -1)
      inverse_y = [self._label_vocab.id2doc(ids) for ids in y]
      if lengths is not None:
          inverse_y = [iy[:l] for iy, l in zip(inverse_y, lengths)]
  
      return inverse_y
  
imports: from ....None import utils
  from ....imports import *
  from .utils import Vocabulary
label_desc: Use image node_id 13 for calling the IndexTransformer obj's underlying member method code with example usage: obj.inverse_transform(y, lengths) and returns: inverse_y