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.
class_docstr_tok: ["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 empty_field(self):
      return IndexField(-1, self.sequence_field.empty_field())
  
file_path: allennlp/allennlp/data/fields/index_field.py
imports: from typing import Dict
  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 4 for calling the IndexField obj's underlying member method code with example usage: obj.empty_field() and returns: IndexField