code: def forward_momentum(self, x):
      key = self.backbone_momentum(x).flatten(start_dim=1)
      key = self.projection_head_momentum(key).detach()
      return key
  
file_path: lightly/examples/pytorch_lightning_distributed/moco.py
imports: import copy
  import pytorch_lightning
  import torch
  import torchvision
  from torch import nn
  from lightly.loss import NTXentLoss
  from lightly.models.modules import MoCoProjectionHead
  from lightly.models.utils import deactivate_requires_grad, update_momentum
  from lightly.transforms.moco_transform import MoCoV2Transform
  from lightly.utils.scheduler import cosine_schedule
label_desc: Use image node_id 3 for calling the MoCo obj's underlying member method code with example usage: obj.forward_momentum(x) and returns: key