text: Title: Can't build the book on MacOS when trying to add MLX implementation
  Body: I'm trying to add Apple's MLX code implementation, but when I am trying to build the html book I run into this when running `d2lbook build html`:
  
  ```
  Traceback (most recent call last):
    File "/Users/zhongkaining/Library/Python/3.9/bin/d2lbook", line 8, in <module>
      sys.exit(main())
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/main.py", line 25, in main
      commands[args.command[0]]()
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 43, in build
      getattr(builder, cmd)()
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 55, in warp
      func(self)
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 342, in html
      self.rst()
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 55, in warp
      func(self)
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 316, in rst
      self.eval()
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 55, in warp
      func(self)
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 160, in eval
      _process_and_eval_notebook(scheduler, src, tgt, run_cells,
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/build.py", line 515, in _process_and_eval_notebook
      scheduler.add(1, num_gpus, target=_job,
    File "/Users/zhongkaining/Library/Python/3.9/lib/python/site-packages/d2lbook/resource.py", line 102, in add
      assert num_cpus <= self._num_cpus and num_gpus <= self._num_gpus, \
  AssertionError: Not enough resources (CPU 2, GPU 0 ) to run the task (CPU 1, GPU 1)
  ```
  
  Looks like building this book requires a GPU, but on MacBook there is no Nvidia GPU. If I want to build on other OS, then I don't MLX will be available on those platforms since this is kinda specifically designed for MacOS.
  
  I think this problem blocks people who attempts to contribute MLX code to this book.