class_docstr: An ``IndexField`` is an index into a
  :class:`~allennlp.data.fields.sequence_field.SequenceField`, as might be used for representing
  a correct answer option in a list, or a span begin and span end position in a passage, for
  example.  Because it's an index into a :class:`SequenceField`, we take one of those as input
  and use it to compute padding lengths.
  
  Parameters
  ----------
  index : ``int``
      The index of the answer in the :class:`SequenceField`.  This is typically the "correct
      answer" in some classification decision over the sequence, like where an answer span starts
      in SQuAD, or which answer option is correct in a multiple choice question.  A value of
      ``-1`` means there is no label, which can be used for padding or other purposes.
  sequence_field : ``SequenceField``
      A field containing the sequence that this ``IndexField`` is a pointer into.
code: def __init__(self, index, sequence_field):
      self.sequence_index = index
      self.sequence_field = sequence_field
  
      if not isinstance(index, int):
          raise ConfigurationError(
              "IndexFields must be passed integer indices. "
              "Found index: {} with type: {}.".format(
                  index, type(index)
              )
          )
  
file_path: magnitude/pymagnitude/third_party/allennlp/data/fields/index_field.py
imports: from __future__ import absolute_import
  import torch
  from allennlp.data.fields.field import Field
  from allennlp.data.fields.sequence_field import SequenceField
  from allennlp.common.checks import ConfigurationError
label_desc: Use image node_id 1 to create a new IndexField object from inherited base classes: Field with example: obj = IndexField(index, sequence_field)