Make the _export function support saver input

[binliunls]

Is your feature request related to a problem? Please describe.
The _export function in the bundle script is for converting models from one format to another one, e.g. converting a pytorch model to a torchscript model. It takes a converter and a model as inputs, converts the given model with the converter and save the converted model to a given path.

However, the saving logic in the _export function just suits to save the torchscript model. Given a converter which exports a torchsciprt model to an onnx model, this logic would fail to save the model. To make the _export function more general, a saver which takes model, savepath, extra_kwargs, should be passed to it instead of the current saving logic.

Describe the solution you'd like
Add a saver to the input parameters of the _export function.
Replace the current saving logic in the _export function with a call of the saver.
Use the _export function to rewrite the onnx_export function.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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