Add nvdiffrast import to mesh_renderer.py for rendering enhancements

trellis/renderers/mesh_renderer.py

@@ -6,6 +6,7 @@
 # distribution of this software and related documentation without an express
 # license agreement from NVIDIA CORPORATION & AFFILIATES is strictly prohibited.
 import torch
+import nvdiffrast.torch as dr
 from easydict import EasyDict as edict
 from ..representations.mesh import MeshExtractResult
 import torch.nn.functional as F