class_docstr: A structure to manage box proposals and their relations with ground truth.
docstr: Returns: #fg indices in [0, N-1] 
docstr_tok: ["Returns",":","#","fg","indices","in","[","0",",","N-1","]"]
code: def fg_inds(self):
      """Returns: #fg indices in [0, N-1]"""
      return tf.reshape(tf.where(self.labels > 0), [-1], name="fg_inds")
  
imports: import tensorflow
  from tensorpack import tfv1
  from tensorpack.models import Conv2D, FullyConnected, layer_register
  from tensorpack.tfutils.argscope import argscope
  from tensorpack.tfutils.scope_utils import under_name_scope
  from tensorpack.tfutils.summary import add_moving_summary
  from tensorpack.utils.argtools import memoized_method
  from config import config
  from utils.box_ops import pairwise_iou
  from .model_box import decode_bbox_target, encode_bbox_target
  from .backbone import GroupNorm
label_desc: Use image node_id 2 for calling the BoxProposals obj's underlying member method code with example usage: obj.fg_inds() and returns: tf