class_docstr: A handle for learners and transformers that share a common indexer.
  
  Lightweight class for pairing a set of independent learners with
  a set of transformers that all share the same cross-validation strategy.
  A :class:`Group` instance is an acceptable caller to
  :class:`~mlens.parallel.ParallelProcessing`.
  
  .. versionadded:: 0.2.0
  
  .. note::
      All instances will share *the same* indexer. If instances have a
      different indexer, that indexer will be replaced.
  
  .. seealso::
      To run a :class:`Group` instance, see :func:`~mlens.parallel.wrapper.run`.
      To handle several groups, use the :class:`~mlens.parallel.layer.Layer`
      class.
  
  Parameters
  ----------
  indexer : inst, optional
      A :obj:`~mlens.index` indexer to build learner and transformers on.
      If not passed, the first indexer of the learners will be enforced
      on all instances.
  
  learners : list, inst, optional
      :class:`~mlens.parallel.learner.Learner` instance(s) attached to
      indexer. Note that :class:`Group` overrides previous
      ``indexer`` parameter settings.
  
  transformers : list, inst, optional
      :class:`~mlens.parallel.learner.Transformer` instance(s) attached to
      indexer. Note that :class:`Group` overrides previous
      ``indexer`` parameter settings.
  
  name : str, optional
      name of group
  
  **kwargs : optional
      Optional keyword arguments to the
      :class:`~mlens.parallel.base.BaseParallel` backend.
code: def __iter__(self):
      # We update optional backend kwargs that might have been passed
      # to ensure these are passed to the instances
      backend_kwargs = {
          param: getattr(self, param)
          for param in ["dtype", "verbose", "raise_on_exception"]
          if hasattr(self, param)
      }
      for tr in self.transformers:
          tr.set_params(**backend_kwargs)
          yield tr
      for lr in self.learners:
          lr.set_params(**backend_kwargs)
          yield lr
  
imports: from .base import BaseEstimator
  from .learner import Learner, Transformer
  from ._base_functions import mold_objects, transform
  from ..utils import format_name, check_instances
  from ..utils.formatting import _check_instances
  from ..externals.sklearn.base import clone, BaseEstimator
label_desc: Use image node_id 2 for calling the Group obj's underlying member method code with example usage: obj.__iter__() without return types