docstr: Calculates perplexity of a given ngram model on a test file.
  
  Args:
      ngram_mod (str): The path to the ngram model file.
      test_far (str): The path to the test file.
  
  Returns:
      str: A string representation of the perplexity calculated.
  
  Raises:
      AssertionError: If the subprocess to calculate perplexity returns a non-zero exit code.
  
  Example:
      >>> perplexity("/path/to/ngram_model", "/path/to/test_file")
      'Perplexity: 123.45'
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code: def perplexity(self, ngram_mod: str, test_far: str) -> str:
      """
      Calculates perplexity of a given ngram model on a test file.
  
      Args:
          ngram_mod (str): The path to the ngram model file.
          test_far (str): The path to the test file.
  
      Returns:
          str: A string representation of the perplexity calculated.
  
      Raises:
          AssertionError: If the subprocess to calculate perplexity returns a non-zero exit code.
  
      Example:
          >>> perplexity("/path/to/ngram_model", "/path/to/test_file")
          'Perplexity: 123.45'
      """
      sh_args = [
          os.path.join(self.ngram_bin_path, "ngramperplexity"),
          "--v=1",
          ngram_mod,
          test_far,
      ]
      ps = subprocess.Popen(
          sh_args,
          text=True,
          stdout=subprocess.PIPE,
          stderr=subprocess.PIPE,
      )
      stdout, stderr = ps.communicate()
      exit_code = ps.wait()
      command = " ".join(sh_args)
      assert (
          exit_code == 0
      ), f"Exit_code must be 0.\n bash command: {command} \n stdout: {stdout} \n stderr: {stderr}"
      perplexity_out = "\n".join(stdout.split("\n")[-6:-1])
      return perplexity_out
  
imports: import argparse
  import os
  import subprocess
  import sys
  from typing import Tuple
  import kenlm_utils
  import torch
  import nemo.collections.asr
  from nemo.collections.asr.modules.rnnt import RNNTDecoder
  from nemo.collections.asr.parts.submodules.ctc_beam_decoding import DEFAULT_TOKEN_OFFSET
  from nemo.utils import logging
label_desc: Use image node_id 5 for calling the NgramMerge obj's underlying member method code with example usage: obj.perplexity(ngram_mod, test_far) and returns: perplexity_out