Enhance Model Export with Custom Saver Functionality

[Han123su]

Fixes #6375 .

Description

Make the _export function focus on model conversion and saving, and add the saver parameter so that different model conversions in the future can also call the _export function, instead of making special behaviors due to specific conditions, but maintaining a consistent behavior pattern.

• Add saver_type.py with various Saver types and Handles specific model parameters using closures.
• Refactor _export function to focus on model conversion and saving, adding a saver parameter for flexibility.
• Rewrite onnx_export、ckpt_export、trt_export function to utilize the updated _export function.
• Update parameter names in save_net_with_metadata for better compatibility.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ ] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.

[Han123su]

Hi @KumoLiu @ericspod, could you please help me review the PR or give some advice to make it better? Thanks for taking the time to check it out.