fall back to CPU if no GPUs are available

[Mayukhdeb]

Fixes the UnboundLocalError that occurs when use_gpu is True but there's no GPUs available on the machine.

Traceback:

Traceback (most recent call last):
  File "/home/cierra/repos/litcoder_core/train_simple.py", line 49, in <module>
    metrics = trainer.train()
              ^^^^^^^^^^^^^^^
  File "/home/cierra/repos/litcoder_core/encoding/trainer.py", line 299, in train
    metrics, weights, best_alphas = self.model.fit_predict(
                                    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/cierra/repos/litcoder_core/encoding/models/nested_cv.py", line 96, in fit_predict
    features_torch = torch.tensor(features, dtype=torch.float32, device=device)
                                                                        ^^^^^^
UnboundLocalError: cannot access local variable 'device' where it is not associated with a value
wandb:
wandb: 🚀 View run abstract-trainer-20251023-135951 at:
wandb: Find logs at: wandb/run-20251023_135951-u7d8ziyj/logs
(litcoder) cierra@xiyarui:~/repos/litcoder_core$