Add "mse" metric option to ALSTM.metric_fn

[raikiriww]

Enhance the metric_fn method of the ALSTM class to include the "mse" metric option.

Description

Add "mse" metric option to ALSTM.metric_fn

Motivation and Context

#1780 (comment)

User can choose "mse" metric to evaluate the model

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:
   Use the file qlib/examples/benchmarks/ALSTM/workflow_config_alstm_Alpha158.yaml for testing. Set task -> model -> kwargs -> metric to mse and set n_jobs to 5 due to the memory limit of my computer.
   

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[you-n-g]

Thanks for your great effort!