Bug: mutable default argument evals_result=dict() in 27 model fit() methods #2167

Description

27 models in qlib/contrib/model/ use evals_result=dict() as a default argument in their fit() method. This is a well-known Python antipattern: the dict is created once at function definition time and shared across all calls, causing potential state leakage between separate fit() invocations.

The correct pattern (already used in gbdt.py and highfreq_gdbt_model.py) is:

def fit(self, dataset, evals_result=None):
    if evals_result is None:
        evals_result = {}

Affected files

All 27 files in qlib/contrib/model/ except gbdt.py and highfreq_gdbt_model.py.

I will submit a fix shortly.