[BUGS] Multi GPU Error after Merging My PR

[ichsan2895]

Sometimes, I create a new feature with new bugs. This repo works well for single GPU when using --use-bilateral-grid and/or --use-fused-bilagrid.

When I use multi-GPU (for example CUDA_VISIBLE_DEVICES=0,1 python3 examples/simple_trainer.py mcmc --use-fused-bilagrid), it gives a brand new error:

Running garden
Distributed worker: 2 / 2
Warning: image_path not found for reconstruction
[Parser] 185 images, taken by 1 cameras.
Downscaling images by 4x from data/360_v2/garden/images to data/360_v2/garden/images_4_png.
Distributed worker: 1 / 2
Warning: image_path not found for reconstruction
[Parser] 185 images, taken by 1 cameras.
Downscaling images by 4x from data/360_v2/garden/images to data/360_v2/garden/images_4_png.
100%|██████████| 185/185 [00:00<00:00, 5715.49it/s]
100%|██████████| 185/185 [00:00<00:00, 5567.73it/s]
Scene scale: 1.2265263065808907
Model initialized. Number of GS: 69383
Scene scale: 1.2265263065808907
Model initialized. Number of GS: 69383
Traceback (most recent call last):
  File "/workspace/GSPLAT_152n3/gsplat-dev/examples/simple_trainer_ErrorReproduction.py", line 1268, in <module>
    cli(main, cfg, verbose=True)
  File "/usr/local/lib/python3.10/dist-packages/gsplat/distributed.py", line 344, in cli
    process_context.join()
  File "/usr/local/lib/python3.10/dist-packages/torch/multiprocessing/spawn.py", line 163, in join
    raise ProcessRaisedException(msg, error_index, failed_process.pid)
torch.multiprocessing.spawn.ProcessRaisedException: 

-- Process 1 terminated with the following error:
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/torch/multiprocessing/spawn.py", line 74, in _wrap
    fn(i, *args)
  File "/usr/local/lib/python3.10/dist-packages/gsplat/distributed.py", line 295, in _distributed_worker
    fn(local_rank, world_rank, world_size, args)
  File "/workspace/GSPLAT_152n3/gsplat-dev/examples/simple_trainer_ErrorReproduction.py", line 1174, in main
    runner = Runner(local_rank, world_rank, world_size, cfg)
  File "/workspace/GSPLAT_152n3/gsplat-dev/examples/simple_trainer_ErrorReproduction.py", line 442, in __init__
    self.bil_grids = BilateralGrid(
NameError: name 'BilateralGrid' is not defined

This PR fixed that error. But my code is a little bit like spaghetti. Feels free for refactoring it.