Fix $volume normalization issue #792
                
     Merged
            
            
          
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Fix: #765
Description
change $volume to $volume+1e-12 such that low volume stocks will not create infinity in features.
Motivation and Context
https://github.com//issues/765 $volume creates infinity for low volume stocks.How Has This Been Tested?
pytest qlib/tests/test_all_pipeline.pyunder upper directory ofqlib.Screenshots of Test Results (if appropriate):
Types of changes