code: def _eval_bkg_samples(self, batch, device):
      bkg_batch = {
          "origin": batch["origin"].clone(),
          "direction": batch["direction"].clone(),
          "near": batch["bkg_near"].clone(),
          "far": batch["bkg_far"].clone(),
      }
      coarse_bkg_samples = ray_utils.ray_to_samples(
          bkg_batch, self.opt.samples_per_ray, perturb=0
      )
      coarse_bkg_pts = coarse_bkg_samples[0].to(device)
      coarse_bkg_dirs = coarse_bkg_samples[1].to(device)
      coarse_bkg_z_vals = coarse_bkg_samples[2].to(device)
      coarse_bkg_out = self.net.coarse_bkg_net(
          coarse_bkg_pts, coarse_bkg_dirs
      )
      coarse_bkg_out = coarse_bkg_out.detach()
  
      _, _, _, coarse_bkg_weights, _ = render_utils.raw2outputs(
          coarse_bkg_out,
          coarse_bkg_z_vals,
          coarse_bkg_dirs[:, 0, :],
          white_bkg=self.opt.white_bkg,
      )
      coarse_bkg_weights = coarse_bkg_weights.detach()
  
      (
          fine_bkg_pts,
          fine_bkg_dirs,
          fine_bkg_z_vals,
      ) = ray_utils.ray_to_importance_samples(
          batch,
          coarse_bkg_z_vals,
          coarse_bkg_weights,
          self.opt.importance_samples_per_ray,
          device=device,
      )
  
      fine_bkg_out = self.net.fine_bkg_net(fine_bkg_pts, fine_bkg_dirs)
      fine_bkg_out = fine_bkg_out.detach()
      return fine_bkg_pts, fine_bkg_dirs, fine_bkg_z_vals, fine_bkg_out
  
imports: import os
  import math
  import time
  import random
  import tqdm
  import torch
  import torch.nn.functional
  import numpy
  import torchvision.utils
  import igl
  import lpips
  from utils import utils, render_utils, ray_utils
  from trainers import tensorboard_helper
  from options import options
  from cameras.captures import ResizedPinholeCapture
  from cameras.pinhole_camera import PinholeCamera
  from models.vanilla import weight_reset
  from utils.constant import HARD_SURFACE_OFFSET, PATCH_SIZE, PATCH_SIZE_SQUARED, CANONICAL_ZOOM_FACTOR, CANONICAL_CAMERA_DIST
label_desc: Use image node_id 4 for calling the HumanNeRFTrainer obj's underlying member method code with example usage: obj._eval_bkg_samples(batch, device) and returns: fine_bkg_pts,  fine_bkg_dirs,  fine_bkg_z_vals,  fine_bkg_out