feat: add batch 3D generation script with VRAM optimization

- Add batch_generate.py: two-phase pipeline (shape→texture) that loads
  models sequentially to avoid OOM on RTX 3080
- Fix mesh_utils.py: make bpy import lazy so load_mesh/save_mesh work
  without Blender installed
- Phase 1: shape generation for all images, then unload
- Phase 2: texture generation for all meshes, then unload
- Skip already-generated outputs for resumability
- Tested: 9/9 images successfully generated textured GLB models

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

hy3dpaint/DifferentiableRenderer/mesh_utils.py

@@ -14,7 +14,6 @@
 
 import os
 import cv2
-import bpy
 import math
 import numpy as np
 from io import StringIO
@@ -197,8 +196,15 @@ def save_mesh(mesh_path, vtx_pos, pos_idx, vtx_uv, uv_idx, texture, metallic=Non
     )
 
 
+def _get_bpy():
+    """Lazy import of bpy (Blender Python API)."""
+    import bpy
+    return bpy
+
+
 def _setup_blender_scene():
     """Setup Blender scene for conversion."""
+    bpy = _get_bpy()
     if "convert" not in bpy.data.scenes:
         bpy.data.scenes.new("convert")
     bpy.context.window.scene = bpy.data.scenes["convert"]
@@ -206,6 +212,7 @@ def _setup_blender_scene():
 
 def _clear_scene_objects():
     """Clear all objects from current Blender scene."""
+    bpy = _get_bpy()
     for obj in bpy.context.scene.objects:
         obj.select_set(True)
         bpy.data.objects.remove(obj, do_unlink=True)
@@ -213,6 +220,7 @@ def _clear_scene_objects():
 
 def _select_mesh_objects():
     """Select all mesh objects in scene."""
+    bpy = _get_bpy()
     bpy.ops.object.select_all(action="DESELECT")
     for obj in bpy.context.scene.objects:
         if obj.type == "MESH":
@@ -224,6 +232,7 @@ def _merge_vertices_if_needed(merge_vertices: bool):
     if not merge_vertices:
         return
 
+    bpy = _get_bpy()
     for obj in bpy.context.selected_objects:
         if obj.type == "MESH":
             bpy.context.view_layer.objects.active = obj
@@ -235,6 +244,7 @@ def _merge_vertices_if_needed(merge_vertices: bool):
 
 def _apply_shading(shade_type: str, auto_smooth_angle: float):
     """Apply shading to selected objects."""
+    bpy = _get_bpy()
     shading_ops = {
         "SMOOTH": lambda: bpy.ops.object.shade_smooth(),
         "FLAT": lambda: bpy.ops.object.shade_flat(),
@@ -247,6 +257,7 @@ def _apply_shading(shade_type: str, auto_smooth_angle: float):
 
 def _apply_auto_smooth(auto_smooth_angle: float):
     """Apply auto smooth based on Blender version."""
+    bpy = _get_bpy()
     angle_rad = math.radians(auto_smooth_angle)
 
     if bpy.app.version < (4, 1, 0):
@@ -266,6 +277,7 @@ def convert_obj_to_glb(
 ) -> bool:
     """Convert OBJ file to GLB format using Blender."""
     try:
+        bpy = _get_bpy()
         _setup_blender_scene()
         _clear_scene_objects()