fix: ensure time is logged even when exception is raised in logt()

[AHS0003]

Motivation and Context

The logt context manager in qlib/log.py places log_cost_time outside the try/finally block. This means if an exception is raised inside a with TimeInspector.logt(): block, the time is never logged and the time mark pushed onto the stack is never popped, causing a stack leak.

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.

Types of changes

• Fix bugs
• Add new feature
• Update documentation