fix(eda): change dtype 'string' to 'object'

[yuzhenmao]

Description

Solved the issue #330, but used a bad method (traverse the dataframe columns before converting it into dd)

How Has This Been Tested?

manually

Snapshots:

No Snapshot

Checklist:

• My code follows the style guidelines of this project
• I have already squashed the commits and make the commit message conform to the project standard.
• I have already marked the commit with "BREAKING CHANGE" or "Fixes #" if needed.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules