class_docstr: Add the "length" of an array along the specified axis.
  
  For example, if target field in the input DataEntry has a shape
  of (32, 24, 3), the axis can be specified as 1 to return 24 for
  the output field. If the target field is a list instead, its
  length will be added.
  
  Parameters
  ----------
  target_field
      Field with target values (array) of time series
  axis
      Axis to output lengths (dimensions) on. Default 0.
  output_field
      Name of newly created field
code: def __init__(
      self,
      target_field: str = FieldName.TARGET,
      axis: int = 0,
      output_field: str = "valid_length",
  ) -> None:
      self.target_field = target_field
      self.axis = axis
      self.output_field = output_field
  
file_path: gluonts/src/gluonts/mx/model/renewal/_transform.py
imports: import numpy
  from gluonts.core.component import validated
  from gluonts.dataset.common import DataEntry
  from gluonts.dataset.field_names import FieldName
  from gluonts.transform import SimpleTransformation
label_desc: Use image node_id 1 to create a new AddAxisLength object from inherited base classes: SimpleTransformation with example: obj = AddAxisLength(target_field, axis, output_field)