Can't pickle local object while training RandLANet on S3DIS

### 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

Can't pickle local object while training RandLANet on S3DIS. I use pytorch.

### Steps to reproduce the bug

```python
import open3d.ml as _ml3d
import open3d.ml.torch as ml3d

model = ml3d.models.RandLANet()

dataset_path = "/Users/kimd999/research/projects/Danny/files/public_dataset/S3DIS/Stanford3dDataset_v1.2_Aligned_Version"
dataset = ml3d.datasets.S3DIS(dataset_path=dataset_path, use_cache=True)

pipeline = ml3d.pipelines.SemanticSegmentation(model=model, dataset=dataset, max_epoch=100)

# prints training progress in the console.
pipeline.run_train()
```

### Error message

INFO - 2022-02-15 13:24:10,927 - semantic_segmentation - DEVICE : cpu
INFO - 2022-02-15 13:24:10,927 - semantic_segmentation - Logging in file : ./logs/RandLANet_S3DIS_torch/log_train_2022-02-15_13:24:10.txt
INFO - 2022-02-15 13:24:10,929 - s3dis - Found 249 pointclouds for train
INFO - 2022-02-15 13:24:10,935 - s3dis - Found 23 pointclouds for validation
INFO - 2022-02-15 13:24:10,937 - semantic_segmentation - Initializing from scratch.
INFO - 2022-02-15 13:24:10,940 - semantic_segmentation - Writing summary in train_log/00003_RandLANet_S3DIS_torch.
INFO - 2022-02-15 13:24:10,940 - semantic_segmentation - Started training
INFO - 2022-02-15 13:24:10,940 - semantic_segmentation - === EPOCH 0/100 ===
training: 0%| | 0/63 [00:00
pipeline.run_train()
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/site-packages/open3d/_ml3d/torch/pipelines/semantic_segmentation.py", line 394, in run_train
for step, inputs in enumerate(tqdm(train_loader, desc='training')):
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/site-packages/tqdm/std.py", line 1180, in __iter__
for obj in iterable:
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 355, in __iter__
return self._get_iterator()
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 301, in _get_iterator
return _MultiProcessingDataLoaderIter(self)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 914, in __init__
w.start()
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
return Popen(process_obj)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
super().__init__(process_obj)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
self._launch(process_obj)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
reduction.dump(process_obj, fp)
File "/Users/kimd999/bin/miniconda3/envs/open3d/lib/python3.8/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'

### Expected behavior

_No response_

### Open3D, Python and System information

```markdown
- Operating system: OSX 10.15.7
- Python version: Python 3.8.12
- Open3D version: open3d version:0.14.1
- System type: x84
- Is this remote workstation?: no
- How did you install Open3D?: pip install open3d
```

### Additional information

_No response_