input: Write a python function to generate a 3D model from a 2D CAD file and return the mesh object.
  assert generate_3d_model('example_cad_file.png') is not None
output: import trimesh
  import numpy as np
  from PIL import Image
  import cv2
  
  def generate_3d_model(cad_file_path):
      # Load the 2D CAD image file
      image = Image.open(cad_file_path).convert('L')
      image = np.array(image)
      
      # Use edge detection to find the contours in the image
      edges = cv2.Canny(image, threshold1=50, threshold2=150)
      contours, _ = cv2.findContours(edges, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
      
      # Create a 3D mesh from the contours
      vertices = []
      for contour in contours:
          for point in contour:
              x, y = point[0]
              vertices.append([x, y, 0])  # Use z = 0 for 2D contours
      
      vertices = np.array(vertices)
      
      # Create faces by connecting consecutive points
      faces = []
      for i in range(len(vertices) - 1):
          faces.append([i, i + 1, i + 2])
      
      faces = np.array(faces)
      
      # Create a Trimesh object from the vertices and faces
      mesh = trimesh.Trimesh(vertices=vertices, faces=faces)
      
      return mesh