Add text file task to T5

add_data_task.py

t5.data.TaskRegistry.add(
      "byt5_ex",
      t5.data.TextLineTask,
      split_to_filepattern={
            "train": "gs://BUCKET/train_lines.txt",
            "validation": "gs://BUCKET/validation_lines.txt",
        },
      text_preprocessor=[
        functools.partial(
          t5.data.preprocessors.parse_tsv,
          field_names=['text'],
          field_delim='~', # check ASCII char doesn't appear in files, default is tab (\t), no \n\n blank lines
        ),
        functools.partial(
              t5.data.preprocessors.rekey,
              key_map={
                  "inputs": None,
                  "targets": "text"
              }),
      ],
      token_preprocessor=functools.partial(
          t5.data.preprocessors.span_corruption,
          mean_noise_span_length=MEAN_NOISE_SPAN_LENGTH),
      output_features=DEFAULT_BYTE_OUTPUT_FEATURES,
      metric_fns=[])