refine: Allow specifying the model config when loading a local model.

[yluuu16]

• problem: PaddleClas loads the config based on model_type during initialization. However, the value of model_type is always set to custom while loading a local model, which results in the default configuration being loaded every time. PaddleClas 初始化时根据 model_type 来加载 config，而使用本地模型时 model_type 在 _check_input_model 中恒为 "custom"，导致使用本地模型一直使用默认配置而无法使用正确的配置文件。
• fix: Allow specifying the model config when loading a local model.

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