Fix unrecognized config bug

[fzc621]

The config key should be checked first instead of assignment, otherwise the if-condition would never be true.

Description

Put the assignment statement after the if-statement.

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

• Fix bugs
• Add new feature
• Update documentation

[you-n-g]

@fzc621 It's great!
Thanks so much!