How to use the gluonts.dataset.field_names.FieldName.START function in gluonts

def test_AddTimeFeatures(start, target, is_train: bool):
    pred_length = 13
    t = transform.AddTimeFeatures(
        start_field=FieldName.START,
        target_field=FieldName.TARGET,
        output_field="myout",
        pred_length=pred_length,
        time_features=[time_feature.DayOfWeek(), time_feature.DayOfMonth()],
    )

    assert_serializable(t)

    data = {"start": start, "target": target}
    res = t.map_transform(data, is_train=is_train)
    mat = res["myout"]
    expected_length = len(target) + (0 if is_train else pred_length)
    assert mat.shape == (2, expected_length)
    tmp_idx = pd.date_range(
        start=start, freq=start.freq, periods=expected_length
    )

def create_transformation(self):
            # Model specific input transform
            # Here we use a transformation that randomly
            # selects training samples from all series.
            return InstanceSplitter(
                target_field=FieldName.TARGET,
                is_pad_field=FieldName.IS_PAD,
                start_field=FieldName.START,
                forecast_start_field=FieldName.FORECAST_START,
                train_sampler=ExpectedNumInstanceSampler(num_instances=1),
                past_length=self.context_length,
                future_length=self.prediction_length,
            )

pred_length=self.prediction_length,
                ),
                transform.VstackFeatures(
                    output_field=FieldName.FEAT_DYNAMIC_REAL,
                    input_fields=[FieldName.FEAT_TIME],
                ),
                transform.SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]
                ),
                transform.AsNumpyArray(
                    field=FieldName.FEAT_STATIC_CAT, expected_ndim=1
                ),
                transform.InstanceSplitter(
                    target_field=FieldName.TARGET,
                    is_pad_field=FieldName.IS_PAD,
                    start_field=FieldName.START,
                    forecast_start_field=FieldName.FORECAST_START,
                    train_sampler=ExpectedNumInstanceSampler(num_instances=1),
                    past_length=self.context_length,
                    future_length=self.prediction_length,
                    time_series_fields=[FieldName.FEAT_DYNAMIC_REAL],
                ),

def create_transformation(
        self, bin_edges: np.ndarray, pred_length: int
    ) -> transform.Transformation:
        return Chain(
            [
                AsNumpyArray(field=FieldName.TARGET, expected_ndim=1),
                AddObservedValuesIndicator(
                    target_field=FieldName.TARGET,
                    output_field=FieldName.OBSERVED_VALUES,
                ),
                AddTimeFeatures(
                    start_field=FieldName.START,
                    target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_TIME,
                    time_features=time_features_from_frequency_str(self.freq),
                    pred_length=self.prediction_length,
                ),
                AddAgeFeature(
                    target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_AGE,
                    pred_length=self.prediction_length,
                ),
                VstackFeatures(
                    output_field=FieldName.FEAT_TIME,
                    input_fields=[FieldName.FEAT_TIME, FieldName.FEAT_AGE],
                ),
                SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]

target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_AGE,
                    pred_length=self.prediction_length,
                ),
                VstackFeatures(
                    output_field=FieldName.FEAT_TIME,
                    input_fields=[FieldName.FEAT_TIME, FieldName.FEAT_AGE],
                ),
                SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]
                ),
                AsNumpyArray(field=FieldName.FEAT_STATIC_CAT, expected_ndim=1),
                InstanceSplitter(
                    target_field=FieldName.TARGET,
                    is_pad_field=FieldName.IS_PAD,
                    start_field=FieldName.START,
                    forecast_start_field=FieldName.FORECAST_START,
                    train_sampler=ExpectedNumInstanceSampler(num_instances=1),
                    past_length=self.context_length,
                    future_length=pred_length,
                    output_NTC=False,
                    time_series_fields=[
                        FieldName.FEAT_TIME,
                        FieldName.OBSERVED_VALUES,
                    ],
                ),
                QuantizeScaled(
                    bin_edges=bin_edges.tolist(),
                    future_target="future_target",
                    past_target="past_target",
                ),

def __init__(
        self,
        past_interval_length: float,
        future_interval_length: float,
        train_sampler: ContinuousTimePointSampler,
        target_field: str = FieldName.TARGET,
        start_field: str = FieldName.START,
        end_field: str = "end",
        forecast_start_field: str = FieldName.FORECAST_START,
    ) -> None:

        assert (
            future_interval_length > 0
        ), "Prediction interval must have length greater than 0."

        self.train_sampler = train_sampler
        self.past_interval_length = past_interval_length
        self.future_interval_length = future_interval_length
        self.target_field = target_field
        self.start_field = start_field
        self.end_field = end_field
        self.forecast_start_field = forecast_start_field

AsNumpyArray(field=FieldName.TARGET, expected_ndim=1),
                AddTimeFeatures(
                    start_field=FieldName.START,
                    target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_TIME,
                    time_features=time_features_from_frequency_str(self.freq),
                    pred_length=self.prediction_length,
                ),
                SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]
                ),
                AsNumpyArray(field=FieldName.FEAT_STATIC_CAT, expected_ndim=1),
                transform.InstanceSplitter(
                    target_field=FieldName.TARGET,
                    is_pad_field=FieldName.IS_PAD,
                    start_field=FieldName.START,
                    forecast_start_field=FieldName.FORECAST_START,
                    train_sampler=TestSplitSampler(),
                    time_series_fields=[FieldName.FEAT_TIME],
                    past_length=self.context_length,
                    future_length=self.prediction_length,
                ),

def create_transformation(self) -> Transformation:
        return Chain(
            [
                AsNumpyArray(field=FieldName.TARGET, expected_ndim=1),
                AddTimeFeatures(
                    start_field=FieldName.START,
                    target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_TIME,
                    time_features=self.time_features,
                    pred_length=self.prediction_length,
                ),
                SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]
                ),
                AsNumpyArray(field=FieldName.FEAT_STATIC_CAT, expected_ndim=1),
                CanonicalInstanceSplitter(
                    target_field=FieldName.TARGET,
                    is_pad_field=FieldName.IS_PAD,
                    start_field=FieldName.START,
                    forecast_start_field=FieldName.FORECAST_START,
                    instance_sampler=TestSplitSampler(),
                    time_series_fields=[FieldName.FEAT_TIME],

def create_transformation(self) -> Transformation:
        return Chain(
            trans=[
                AsNumpyArray(field=FieldName.TARGET, expected_ndim=1),
                AddTimeFeatures(
                    start_field=FieldName.START,
                    target_field=FieldName.TARGET,
                    output_field=FieldName.FEAT_TIME,
                    time_features=time_features_from_frequency_str(self.freq),
                    pred_length=self.prediction_length,
                ),
                SetFieldIfNotPresent(
                    field=FieldName.FEAT_STATIC_CAT, value=[0.0]
                ),
                AsNumpyArray(field=FieldName.FEAT_STATIC_CAT, expected_ndim=1),
                InstanceSplitter(
                    target_field=FieldName.TARGET,
                    is_pad_field=FieldName.IS_PAD,
                    start_field=FieldName.START,
                    forecast_start_field=FieldName.FORECAST_START,
                    train_sampler=TestSplitSampler(),
                    time_series_fields=[FieldName.FEAT_TIME],

week_dict = {
        0: "MON",
        1: "TUE",
        2: "WED",
        3: "THU",
        4: "FRI",
        5: "SAT",
        6: "SUN",
    }
    for i in range(len(time_series)):
        data = time_series[i]
        timestamp = pd.Timestamp(data[FieldName.START])
        freq_week_start = freq
        if freq_week_start == "W":
            freq_week_start = f"W-{week_dict[timestamp.weekday()]}"
        timestamp = pd.Timestamp(data[FieldName.START], freq=freq_week_start)
        item_id = int(data[FieldName.ITEM_ID])
        for j, target in enumerate(data[FieldName.TARGET]):
            # Using convention that there are no missing values before the start date
            if is_missing and j != 0 and j % num_missing == 0:
                timestamp += 1
                continue  # Skip every 4th entry
            else:
                timestamp_row = timestamp
                if freq in ["W", "D", "M"]:
                    timestamp_row = timestamp.date()
                row = [item_id, timestamp_row, target]
                # Check if related time series is present
                if FieldName.FEAT_DYNAMIC_REAL in data.keys():
                    for feat_dynamic_real in data[FieldName.FEAT_DYNAMIC_REAL]:
                        row.append(feat_dynamic_real[j])
                csv_writer.writerow(row)