docs: add some details for tutorials of fine-tuning

[XixinYang]

Thank you for your contribution to the MindCV repo. Before submitting this PR, please make sure:

• [x] You have read the Contributing Guidelines on pull requests
• [x] Your code builds clean without any errors or warnings
• [x] You are using approved terminology
• [ ] You have added unit tests

Motivation

①删除了finetune.py文件中不必要的关于断点续训的内容； ②限定了finetune教程的示例场景为基于mindcv预训练模型的微调，同时注明了使用本地ckpt时参数的设置； ③注明 了ckpt中分类层参数的加载规则；

Test Plan

(How should this PR be tested? Do you require special setup to run the test or repro the fixed bug?)

Related Issues and PRs

(Is this PR part of a group of changes? Link the other relevant PRs and Issues here. Use https://help.github.com/en/articles/closing-issues-using-keywords for help on GitHub syntax)