class_docstr: A MapTransformFn that converts input blocks to rows.
class_docstr_tok: ["A","MapTransformFn","that","converts","input","blocks","to","rows","."]
docstr: Returns the singleton instance.
docstr_tok: ["Returns","the","singleton","instance","."]
code: def instance(cls) -> "BlocksToRowsMapTransformFn":
      """Returns the singleton instance."""
      if getattr(cls, "_instance", None) is None:
          cls._instance = cls()
      return cls._instance
  
imports: import itertools
  from abc import abstractmethod
  from enum import Enum
  from typing import Any, Callable, Dict, Iterable, List, Optional, TypeVar, Union
  from ray.data._internal.block_batching.block_batching import batch_blocks
  from ray.data._internal.execution.interfaces.task_context import TaskContext
  from ray.data._internal.output_buffer import BlockOutputBuffer
  from ray.data.block import Block, BlockAccessor, DataBatch
label_desc: Use image node_id 3 for calling the BlocksToRowsMapTransformFn obj's underlying member method code with example usage: obj.instance(cls) and returns: cls