Fix Error in Accuracy Tests for Multi-Label Classification.

[chakravarthik27]

File "/nonexistent/src/model_testing.py", line 38, in <module>
    tester.run_model_testing()
  File "/nonexistent/src/model_test_base.py", line 427, in run_model_testing
    self.run_test()
  File "/nonexistent/src/model_test_base.py", line 366, in run_test
    self.harness.run()
  File "/usr/local/lib/python3.10/dist-packages/langtest/langtest.py", line 319, in run
    self._generated_results = self.__single_dataset_run(
  File "/usr/local/lib/python3.10/dist-packages/langtest/langtest.py", line 1656, in __single_dataset_run
    generated_results = TestFactory.run(
  File "/usr/local/lib/python3.10/dist-packages/langtest/transform/base.py", line 148, in run
    temp_res = asyncio.run(async_tests)
  File "/usr/local/lib/python3.10/dist-packages/nest_asyncio.py", line 30, in run
    return loop.run_until_complete(task)
  File "/usr/local/lib/python3.10/dist-packages/nest_asyncio.py", line 98, in run_until_complete
    return f.result()
  File "/usr/lib/python3.10/asyncio/futures.py", line 201, in result
    raise self._exception.with_traceback(self._exception_tb)
  File "/usr/lib/python3.10/asyncio/tasks.py", line 232, in __step
    result = coro.send(None)
  File "/usr/local/lib/python3.10/dist-packages/langtest/transform/base.py", line 198, in async_run
    category_output = all_categories[each].run(
  File "/usr/local/lib/python3.10/dist-packages/langtest/transform/accuracy.py", line 170, in run
    y_pred = y_pred.apply(lambda x: x[0])
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/series.py", line 4924, in apply
    ).apply()
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/apply.py", line 1427, in apply
    return self.apply_standard()
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/apply.py", line 1507, in apply_standard
    mapped = obj._map_values(
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/base.py", line 921, in _map_values
    return algorithms.map_array(arr, mapper, na_action=na_action, convert=convert)
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/algorithms.py", line 1743, in map_array
    return lib.map_infer(values, mapper, convert=convert)
  File "lib.pyx", line 2972, in pandas._libs.lib.map_infer
  File "/usr/local/lib/python3.10/dist-packages/langtest/transform/accuracy.py", line 170, in <lambda>
    y_pred = y_pred.apply(lambda x: x[0])
IndexError: list index out of range