code: def test_sample_vertex_textures_grad(self):
      verts = torch.randn((4, 3), dtype=torch.float32)
      faces = torch.tensor([[2, 1, 0], [3, 1, 0]], dtype=torch.int64)
      vert_tex = torch.tensor(
          [[0, 1, 0], [0, 1, 1], [1, 1, 0], [1, 1, 1]],
          dtype=torch.float32,
          requires_grad=True,
      )
      verts_features = vert_tex
      tex = TexturesVertex(verts_features=[verts_features])
      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),
      )
      grad_vert_tex = torch.tensor(
          [
              [0.3, 0.3, 0.3],
              [0.9, 0.9, 0.9],
              [0.5, 0.5, 0.5],
              [0.3, 0.3, 0.3],
          ],
          dtype=torch.float32,
      )
      texels = mesh.sample_textures(fragments)
      texels.sum().backward()
      self.assertTrue(hasattr(vert_tex, "grad"))
      self.assertTrue(
          torch.allclose(vert_tex.grad, grad_vert_tex[None, :])
      )
  
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 TestTexturesVertex obj's underlying member method code with example usage: obj.test_sample_vertex_textures_grad() without return types