add benchmark for LSTM on csi500

[jianyexi]

Description

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