Set id_mapping through text file

[illian01]

Description

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Part of #426

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Change(s)

• id_mapping can be text file path (relative path from root) for classification and detection tasks.
• Update for segmentation and pose estimation tasks will be done later.

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- Added a new feature by `@myusername` in [PR 2023](https://github.com/Nota-NetsPresso/netspresso-trainer/pull/2023)