message: Update Docs and implement test for negative blank_index
diff: @@ -246,6 +246,7 @@ class CTCGreedyDecoderTest(test.TestCase):
  [2, 3], dtype=np.int64)),
  ]
  + # Test positive value in blank_index
  self._testCTCDecoder(
  ctc_ops.ctc_greedy_decoder,
  inputs,
  @@ -254,6 +255,15 @@ class CTCGreedyDecoderTest(test.TestCase):
  decode_truth,
  blank_index=2)
  + # Test negative value in blank_index
  + self._testCTCDecoder(
  + ctc_ops.ctc_greedy_decoder,
  + inputs,
  + seq_lens,
  + log_prob_truth,
  + decode_truth,
  + blank_index=-2)
  +
  @test_util.run_deprecated_v1
  def testCTCDecoderBeamSearch(self):
  """Test one batch, two beams - hibernating beam search."""
  