feat: only one api key

[SAKURA-CAT]

Description

This pull request includes several changes to the swanlab/package.py file and the associated unit tests to improve the handling of .netrc files and ensure that only one host entry is maintained. The most important changes include the addition of a new function to get the .netrc file path, updates to existing functions to use this new function, and enhancements to the unit tests to verify the new behavior.

Enhancements to .netrc file handling:

• swanlab/package.py: Added a new function get_nrc_path() to obtain the .netrc file path.
• swanlab/package.py: Updated the get_key() and save_key() functions to use the new get_nrc_path() function for obtaining the .netrc file path. [1] [2]
• swanlab/package.py: Modified the save_key() function to avoid writing duplicate host entries and ensure only one host entry is maintained.

Improvements to unit tests:

• test/unit/test_package.py: Enhanced the test_ok test to verify that only one host entry is maintained after saving a new host.
• test/unit/test_package.py: Added a new test test_duplicate to check the behavior of saving duplicate entries, ensuring that the .netrc file is not modified unnecessarily.

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closes #797