BUG: Allow pairwise calcuation when comparing the column with itself (pandas-dev#25781)

Author: liwh

pandas/core/frame.py

@@ -9396,6 +9396,7 @@ def corr(
         self,
         method: str | Callable[[np.ndarray, np.ndarray], float] = "pearson",
         min_periods: int = 1,
+        calculate_diagonal: bool = False
     ) -> DataFrame:
         """
         Compute pairwise correlation of columns, excluding NA/null values.
@@ -9465,7 +9466,7 @@ def corr(
                     valid = mask[i] & mask[j]
                     if valid.sum() < min_periods:
                         c = np.nan
-                    elif i == j:
+                    elif i == j and not calculate_diagonal:
                         c = 1.0
                     elif not valid.all():
                         c = corrf(ac[valid], bc[valid])