support finding bert-base-uncased in extra model paths

[Maslino]

Currently, bert-base-uncased directory must be placed under ComfyUI/models directory. This PR supports to search bert-base-uncased directory in extra model paths which can be specified in ComfyUI's extra_model_paths.yaml like this

public:
    base_path: your-models-dir
    bert-base-uncased: bert-base-uncased