code: def __call__(self, t: int, u: int, v: Optional[int] = None):
      # if indexing all the values of the vocabulary, then only t, u are provided
      if v is None:
          return t * self.U + u
      else:
          # otherwise, t, u, v are provided to index particular value
          # in the vocabulary.
          if self.batch_first:
              return (t * self.maxU + u) * self.alphabet_size + v
          else:
              return (
                  t * self.maxU + u
              ) * self.minibatch * self.alphabet_size + v
  
imports: import math
  import multiprocessing
  from typing import Optional
  import numba
  import torch
  from torch.autograd import Function
  from espnet2.asr.transducer.rnnt_multi_blank.utils import global_constants
label_desc: Use image node_id 2 for calling the CpuRNNT_index obj's underlying member method code with example usage: obj.__call__(t, u, v) and returns: unknown, unknown, unknown