AttributeError: Can't pickle local object 'SemSegRandomSampler.get_point_sampler.<locals>._random_centered_gen'

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

When I run RandLA-Net on Toronto3D dataset, I met this bug. I seems the multiprocesing error, have you ever met before?

AttributeError: Can't pickle local object 'SemSegRandomSampler.get_point_sampler.._random_centered_gen'
[W CudaIPCTypes.cpp:15] Producer process has been terminated before all shared CUDA tensors released. See Note [Sharing CUDA tensors]

### Steps to reproduce the bug

```python
training: 0%| | 0/50 [00:00
sys.exit(main())
File "/export/home2/hanxiaobing/Documents/Open3D-ML-code/Open3D-ML/scripts/run_pipeline.py", line 180, in main
pipeline.run_train()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/open3d/_ml3d/torch/pipelines/semantic_segmentation.py", line 406, in run_train
for step, inputs in enumerate(tqdm(train_loader, desc='training')):
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/tqdm/std.py", line 1195, in __iter__
for obj in iterable:
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 438, in __iter__
return self._get_iterator()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 384, in _get_iterator
return _MultiProcessingDataLoaderIter(self)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 1048, in __init__
w.start()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/context.py", line 291, in _Popen
return Popen(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_forkserver.py", line 35, in __init__
super().__init__(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_fork.py", line 19, in __init__
self._launch(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_forkserver.py", line 47, in _launch
reduction.dump(process_obj, buf)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object 'SemSegRandomSampler.get_point_sampler.._random_centered_gen'
[W CudaIPCTypes.cpp:15] Producer process has been terminated before all shared CUDA tensors released. See Note [Sharing CUDA tensors
```

### Error message

training: 0%| | 0/50 [00:00
sys.exit(main())
File "/export/home2/hanxiaobing/Documents/Open3D-ML-code/Open3D-ML/scripts/run_pipeline.py", line 180, in main
pipeline.run_train()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/open3d/_ml3d/torch/pipelines/semantic_segmentation.py", line 406, in run_train
for step, inputs in enumerate(tqdm(train_loader, desc='training')):
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/tqdm/std.py", line 1195, in __iter__
for obj in iterable:
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 438, in __iter__
return self._get_iterator()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 384, in _get_iterator
return _MultiProcessingDataLoaderIter(self)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 1048, in __init__
w.start()
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/context.py", line 291, in _Popen
return Popen(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_forkserver.py", line 35, in __init__
super().__init__(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_fork.py", line 19, in __init__
self._launch(process_obj)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/popen_forkserver.py", line 47, in _launch
reduction.dump(process_obj, buf)
File "/export/home2/hanxiaobing/anaconda3/envs/Open3D-ML-Pytorch/lib/python3.10/multiprocessing/reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object 'SemSegRandomSampler.get_point_sampler.._random_centered_gen'
[W CudaIPCTypes.cpp:15] Producer process has been terminated before all shared CUDA tensors released. See Note [Sharing CUDA tensors

### Expected behavior

_No response_

### Open3D, Python and System information

```markdown
CUDA 11.6
Python 3.10
Ubuntu 20.04
```

### Additional information

_No response_