训练encoder的报错，已经进行音频和梅尔频谱的预处理

**Summary[问题简述（一句话）]**
1、Exception: Caught Exception in DataLoader worker process 0.
2、Exception: Can't create RandomCycler from an empty collection

**Env & To Reproduce[复现与环境]**
模型是新拉的程序里面自带的encoder/saved_model/pretrained.pt

**Screenshots[截图（如有）]**


Start by reproducing the DataLoader worker error using the bundled encoder/saved_model/pretrained.pt and inspect the audio and mel-spectrogram preprocessing inputs. Trace where RandomCycler receives an empty collection; done when encoder training completes without that exception and the affected preprocessing case is covered.

Written by the indexing model from the issue text.