class_docstr: A helper model so we can manange models more easily. It contains net def
  and parameter storages. You can add an Operator yourself, e.g.
  
      model = model_helper.ModelHelper(name="train_net")
      # init your weight and bias as w and b
      w = model.param_init_net.XavierFill(...)
      b = model.param_init_net.ConstantFill(...)
      fc1 = model.FC([input, w, b], output, **kwargs)
  
  or you can use helper functions in brew module without manually
  defining parameter initializations and operators.
  
      model = model_helper.ModelHelper(name="train_net")
      fc1 = brew.fc(model, input, output, dim_in, dim_out, **kwargs)
code: def AddGradientOperators(self, *args, **kwargs):
      if self.gradient_ops_added:
          raise RuntimeError(
              "You cannot run AddGradientOperators twice."
          )
      self.Validate()
  
      self.gradient_ops_added = True
      self.grad_map = self.net.AddGradientOperators(*args, **kwargs)
      self.param_to_grad = self.get_param_to_grad(self.params)
  
      # Populate ParameterInfo for all parameters if missing
      # and add gradient blob information. So optimizers can use it
      for param, grad in self.param_to_grad.items():
          param_info = self.get_param_info(param)
          if param_info:
              param_info.grad = grad
          else:
              self._parameters_info[
                  param
              ] = parameter_info.ParameterInfo(
                  param_id=None,
                  param=param,
                  grad=grad,
              )
  
      return self.grad_map
  
imports: from caffe2.python import core, scope, workspace
  from caffe2.python.helpers.db_input import db_input
  from caffe2.python.modeling import parameter_info
  from caffe2.python.modeling.parameter_sharing import parameter_sharing_context
  from caffe2.python.optimizer_context import OptimizerContext, DEFAULT_OPTIM
  from caffe2.python.regularizer_context import RegularizerContext
  from itertools import chain
  import logging
label_desc: Use image node_id 17 for calling the ModelHelper obj's underlying member method code with example usage: obj.AddGradientOperators() and returns: self