fix(ops): make PairRolling input assert actually check its inputs

[UditDewan]

Description

The input-validation assert in PairRolling._load_internal (qlib/data/ops.py) can never fail:

assert any(
    [isinstance(self.feature_left, Expression), self.feature_right, Expression]
), "at least one of two inputs is Expression instance"

The second element is the raw feature_right value and the third is the Expression class object, which is always truthy — so any(...) is always True. The intended check was clearly "at least one of the two inputs is an Expression instance" (per the message). As a result, Cov(1.0, 2.0, 3) sails past the assert and later fails with an unrelated AttributeError: 'float' object has no attribute 'rolling'.

This PR replaces it with the intended check:

assert isinstance(self.feature_left, Expression) or isinstance(
    self.feature_right, Expression
), "at least one of two inputs is Expression instance"

Motivation and Context

Invalid PairRolling inputs (e.g. Corr/Cov built with two constants) should fail fast with the intended message instead of surfacing a confusing AttributeError deep in pandas. No valid usage changes behavior: any call with at least one Expression input passes the assert exactly as before.

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.

Added tests/ops/test_pair_rolling_inputs.py (no data download needed — uses a stub Expression):

• two non-Expression inputs now raise AssertionError (previously: passed the assert, crashed later with AttributeError)
• Cov of two expressions still matches pandas.Series.rolling().cov()

Screenshots of Test Results (if appropriate):

1. Pipeline test (run with MLFLOW_ALLOW_FILE_STORE=true, since mlflow 3.14 gates the ./mlruns file store behind that flag independently of this change):

tests/test_all_pipeline.py::TestAllFlow::test_0_train PASSED
tests/test_all_pipeline.py::TestAllFlow::test_1_backtest PASSED
tests/test_all_pipeline.py::TestAllFlow::test_2_expmanager PASSED
================ 3 passed, 1351 warnings in 247.62s (0:04:07) =================

2. Your own tests:

tests/ops/test_pair_rolling_inputs.py::TestPairRollingInputCheck::test_expression_inputs_accepted PASSED
tests/ops/test_pair_rolling_inputs.py::TestPairRollingInputCheck::test_two_numeric_inputs_rejected PASSED
============================== 2 passed in 1.95s ==============================

Without the fix, test_two_numeric_inputs_rejected fails with AttributeError: 'float' object has no attribute 'rolling' at qlib/data/ops.py:1432.

Types of changes

• Fix bugs
• Add new feature
• Update documentation