class_docstr: Serializer for HDF5 format.
  
  This is the standard serializer in Chainer. The chain hierarchy is simply
  mapped to HDF5 hierarchical groups.
  
  Args:
      group (h5py.Group): The group that this serializer represents.
      compression (int): Gzip compression level.
code: def __call__(self, key, value):
      if value is None:
          # use Empty to represent None
          if h5py.version.version_tuple < (2, 7, 0):
              raise RuntimeError(
                  "h5py>=2.7.0 is required to serialize None."
              )
          arr = h5py.Empty("f")
          compression = None
      else:
          arr = _cpu._to_cpu(value)
          compression = None if arr.size <= 1 else self.compression
      self.group.create_dataset(key, data=arr, compression=compression)
      return value
  
imports: import sys
  import numpy
  import six
  from chainer.backends import _chainerx
  from chainer.backends import _cpu
  from chainer.backends import cuda
  from chainer.backends import intel64
  from chainer import serializer
  import chainerx
label_desc: Use image node_id 3 for calling the HDF5Serializer obj's underlying member method code with example usage: obj.__call__(key, value) and returns: value