fix(ops): Corr crashes in expanding mode (N=0)

[UditDewan]

Description

Corr(x, y, 0) (expanding mode) crashes with ValueError: min_periods 1 must be <= window 0.

PairRolling documents N=0 as expanding mode (same convention as Rolling), and Cov(x, y, 0) works. But Corr._load_internal applies its zero-std NaN mask with an unconditional series.rolling(self.N, min_periods=1).std(), and pandas rejects rolling(0):

File "qlib/data/ops.py", line 1495, in _load_internal
    np.isclose(series_left.rolling(self.N, min_periods=1).std(), 0, atol=2e-05)
...
ValueError: min_periods 1 must be <= window 0

This PR computes the mask with expanding(min_periods=1).std() when N == 0 and keeps the rolling path byte-for-byte identical for N > 0.

Motivation and Context

Any expression using expanding correlation, e.g. Corr($close, ChangeInstrument('SH000300', $close), 0), currently fails, while the equivalent Cov(..., 0) succeeds. The masking line was added to suppress spurious correlations of near-constant series and simply never handled the expanding branch.

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.py (no data download needed — uses a stub Expression):

• expanding Corr matches pandas.Series.expanding().corr() (previously: crash)
• constant series are still masked to NaN in expanding mode
• rolling (N > 0) results are unchanged

Screenshots of Test Results (if appropriate):

1. Your own tests:

test_corr_expanding_masks_constant_series ... ok
test_corr_expanding_no_crash ... ok
test_corr_rolling_unchanged ... ok

----------------------------------------------------------------------
Ran 3 tests in 0.007s

OK

Types of changes

• Fix bugs
• Add new feature
• Update documentation