class_docstr: A splitter that slices training and test data based on a fixed integer
  offset.
  
  Parameters
  ----------
  offset
      Offset determining where the training data ends.
      A positive offset indicates how many observations since the start of
      each series should be in the training slice; a negative offset
      indicates how many observations before the end of each series should
      be excluded from the training slice.
code: def training_entry(self, entry: DataEntry) -> DataEntry:
      return slice_data_entry(entry, slice(None, self.offset))
  
imports: from abc import ABC, abstractmethod
  from dataclasses import dataclass
  from typing import Generator, Optional, Tuple
  import pandas
  from gluonts.dataset import Dataset, DataEntry
  from gluonts.dataset.field_names import FieldName
label_desc: Use image node_id 1 for calling the OffsetSplitter obj's underlying member method code with example usage: obj.training_entry(entry) and returns: slice_data_entry