code: def test_textures_atlas_grad(self):
      N, F, R = 1, 2, 2
      verts = torch.randn((4, 3), dtype=torch.float32)
      faces = torch.tensor([[2, 1, 0], [3, 1, 0]], dtype=torch.int64)
      faces_atlas = torch.rand(size=(N, F, R, R, 3), requires_grad=True)
      tex = TexturesAtlas(atlas=faces_atlas)
      mesh = Meshes(verts=[verts], faces=[faces], textures=tex)
      pix_to_face = torch.tensor([0, 1], dtype=torch.int64).view(
          1, 1, 1, 2
      )
      barycentric_coords = torch.tensor(
          [[0.5, 0.3, 0.2], [0.3, 0.6, 0.1]], dtype=torch.float32
      ).view(1, 1, 1, 2, -1)
      fragments = Fragments(
          pix_to_face=pix_to_face,
          bary_coords=barycentric_coords,
          zbuf=torch.ones_like(pix_to_face),
          dists=torch.ones_like(pix_to_face),
      )
      texels = mesh.textures.sample_textures(fragments)
      grad_tex = torch.rand_like(texels)
      grad_expected = torch.zeros_like(faces_atlas)
      grad_expected[0, 0, 0, 1, :] = grad_tex[..., 0:1, :]
      grad_expected[0, 1, 1, 0, :] = grad_tex[..., 1:2, :]
      texels.backward(grad_tex)
      self.assertTrue(hasattr(faces_atlas, "grad"))
      self.assertTrue(torch.allclose(faces_atlas.grad, grad_expected))
  
imports: import unittest
  import torch
  import torch.nn.functional
  from pytorch3d.renderer.mesh.rasterizer import Fragments
  from pytorch3d.renderer.mesh.textures import _list_to_padded_wrapper, TexturesAtlas, TexturesUV, TexturesVertex
  from pytorch3d.renderer.mesh.utils import pack_rectangles, pack_unique_rectangles, Rectangle
  from pytorch3d.structures import list_to_packed, Meshes, packed_to_list
  from .common_testing import TestCaseMixin
  from .test_meshes import init_mesh
label_desc: Use image node_id 2 for calling the TestTexturesAtlas obj's underlying member method code with example usage: obj.test_textures_atlas_grad() without return types