fix: fix a bug in KGFBWorkspace (about data loading)

[WinstonLiyt]

Description

Fix a bug in KGFBWorkspace.

Motivation and Context

How Has This Been Tested?

• [ ] Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• [ ] If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• [x] Fix bugs
• [ ] Add new feature
• [ ] Update documentation

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📚 Documentation preview 📚: https://RDAgent--495.org.readthedocs.build/en/495/