class_docstr: Sum of marginal log likelihoods, to be used with Multi-Output models.
  
  Args:
      likelihood: A MultiOutputLikelihood
      model: A MultiOutputModel
      mll_cls: The Marginal Log Likelihood class (default: ExactMarginalLogLikelihood)
  
  In case the model outputs are independent, this provives the MLL of the multi-output model.
code: def __init__(
      self, likelihood, model, mll_cls=ExactMarginalLogLikelihood
  ):
      super().__init__(model.likelihood, model)
      self.mlls = ModuleList(
          [mll_cls(mdl.likelihood, mdl) for mdl in model.models]
      )
  
imports: from torch.nn import ModuleList
  from gpytorch.mlls import ExactMarginalLogLikelihood, MarginalLogLikelihood
  from gpytorch.utils.generic import length_safe_zip
label_desc: Use image node_id 1 to create a new SumMarginalLogLikelihood object from inherited base classes: MarginalLogLikelihood with example: obj = SumMarginalLogLikelihood(likelihood, model, mll_cls)