[ez] add docblock for _expandsums (#154397)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/154397
Approved by: https://github.com/laithsakka
ghstack dependencies: #154400, #154398, #154396, #154399

torch/fx/experimental/symbolic_shapes.py

@@ -2161,6 +2161,22 @@ IndicatorTypes = (IsNonOverlappingAndDenseIndicator,)
 
 
 def _expandsums(args: list[sympy.Expr]) -> tuple[sympy.Expr, bool]:
+    """
+    Expand products of sums into sums of products.
+
+    This function takes a list of sympy expressions and separates them into
+    additive expressions (those with is_Add=True) and other expressions.
+    It then computes the distributive product, expanding (a+b)*(c+d) into a*c + a*d + b*c + b*d.
+
+    Args:
+        args: A list of sympy expressions to expand
+
+    Returns:
+        A tuple containing:
+        - The expanded expression as a sympy.Expr
+        - A boolean indicating whether expansion occurred (True if multiple additive
+          expressions were present or if there was at least one additive and one other expression)
+    """
     adds, other = [], []
     for arg in args:
         if arg.is_Add: