This attribute is not directly used in mobilenet_v2, but in FasterRCNN.
From the docs for FasterRCNN:

backbone (nn.Module): the network used to compute the features for the model.
It should contain a out_channels attribute, which indicates the number of output
channels that each feature map has (and it should be the same for all feature maps).
The backbone should return a single Tensor or and OrderedDict[Tensor].

So you are just creating this attribute, so that it can be used in the RPNHead.