RuntimeError: weight tensor should be defined either for all 8 classes or no classes but got weight tensor of shape: [1, 8]

### 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 `main` branch).

### My Question

****I trained my own data before and got good results and it worked fine. However, when I try to retrain the same data with the same parameters a few months later, I get the following error. What could be the reason and solution?****

RuntimeError Traceback (most recent call last)
Cell In[3], line 18
15 pipeline = SemanticSegmentation(model=model, dataset=dataset, **cfg.pipeline , device='cuda') #max_epoch=100,
17 # prints training progress in the console.
---> 18 pipeline.run_train()

File ~/Masaüstü/Open3D-ML-0.17.0/ml3d/torch/pipelines/semantic_segmentation.py:410, in SemanticSegmentation.run_train(self)
408 self.optimizer.zero_grad()
409 results = model(inputs['data'])
--> 410 loss, gt_labels, predict_scores = model.get_loss(
411 Loss, results, inputs, device)
413 if predict_scores.size()[-1] == 0:
414 continue

File ~/Masaüstü/Open3D-ML-0.17.0/ml3d/torch/models/randlanet.py:378, in RandLANet.get_loss(self, Loss, results, inputs, device)
373 labels = inputs['data']['labels']
375 scores, labels = filter_valid_label(results, labels, cfg.num_classes,
376 cfg.ignored_label_inds, device)
--> 378 loss = Loss.weighted_CrossEntropyLoss(scores, labels)
380 return loss, labels, scores

File ~/anaconda3/envs/bimtas/lib/python3.9/site-packages/torch/nn/modules/module.py:1194, in Module._call_impl(self, *input, **kwargs)
1190 # If we don't have any hooks, we want to skip the rest of the logic in
1191 # this function, and just call forward.
1192 if not (self._backward_hooks or self._forward_hooks or self._forward_pre_hooks or _global_backward_hooks
1193 or _global_forward_hooks or _global_forward_pre_hooks):
-> 1194 return forward_call(*input, **kwargs)
1195 # Do not call functions when jit is used
1196 full_backward_hooks, non_full_backward_hooks = [], []

File ~/anaconda3/envs/bimtas/lib/python3.9/site-packages/torch/nn/modules/loss.py:1174, in CrossEntropyLoss.forward(self, input, target)
1173 def forward(self, input: Tensor, target: Tensor) -> Tensor:
-> 1174 return F.cross_entropy(input, target, weight=self.weight,
1175 ignore_index=self.ignore_index, reduction=self.reduction,
1176 label_smoothing=self.label_smoothing)

File ~/anaconda3/envs/bimtas/lib/python3.9/site-packages/torch/nn/functional.py:3026, in cross_entropy(input, target, weight, size_average, ignore_index, reduce, reduction, label_smoothing)
3024 if size_average is not None or reduce is not None:
3025 reduction = _Reduction.legacy_get_string(size_average, reduce)
-> 3026 return torch._C._nn.cross_entropy_loss(input, target, weight, _Reduction.get_enum(reduction), ignore_index, label_smoothing)

RuntimeError: weight tensor should be defined either for all 8 classes or no classes but got weight tensor of shape: [1, 8]