Replace colon with dash in timestamp variables

[Cassius0924]

When running the Semantic Segmentation program on exFAT SSD, Python cannot create log files with colon in the filename timestamp. This causes OSError.

test 0/1: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 79845/79845 [00:06<00:00, 13174.09it/s]
/root/.virtualenvs/ETRS/lib/python3.8/site-packages/open3d/_ml3d/torch/modules/metrics/semseg_metric.py:54: RuntimeWarning: Mean of empty slice
  accs.append(np.nanmean(accs))
/root/.virtualenvs/ETRS/lib/python3.8/site-packages/open3d/_ml3d/torch/modules/metrics/semseg_metric.py:87: RuntimeWarning: Mean of empty slice
  ious.append(np.nanmean(ious))
Traceback (most recent call last):
  File "SemanticSegmentation.py", line 43, in <module>
    pipeline.run_test()
  File "/root/.virtualenvs/ETRS/lib/python3.8/site-packages/open3d/_ml3d/torch/pipelines/semantic_segmentation.py", line 198, in run_test
    log.addHandler(logging.FileHandler(log_file_path))
  File "/usr/lib/python3.8/logging/__init__.py", line 1147, in __init__
    StreamHandler.__init__(self, self._open())
  File "/usr/lib/python3.8/logging/__init__.py", line 1176, in _open
    return open(self.baseFilename, self.mode, encoding=self.encoding)
OSError: [Errno 22] Invalid argument: '/home/ncistwlwsys/hezhizhou-projects/disk/SingleAzureKinect3DReconstruction/src/python/logs/RandLANet_SemanticKITTI_torch/log_test_2023-10-19_20:57:44.txt'

Verified that with this fix, the Semantic Segmentation program can now successfully create log files on exFAT SSD.

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This change is 

[ssheorey]

Thanks @Cassius0924 !