[ET] Fix serde handling of inf

Differential Revision: D65848116

Pull Request resolved: #6799

exir/serde/export_serialize.py

@@ -1847,7 +1847,13 @@ def deserialize(
             self.symbol_name_to_range = {}
             if symbol_name_to_range:
                 for k, vr in symbol_name_to_range.items():
-                    lower = int(vr.lower)
+                    if math.isinf(vr.lower) and vr.lower < 0:
+                        lower = -math.inf
+                    elif math.isinf(vr.lower):
+                        lower = math.inf
+                    else:
+                        lower = int(vr.lower)
+
                     if vr.upper >= 2:  # max is >= 2, not sym bool range
                         lower = max(2, lower)
                     self.symbol_name_to_range[k] = symbolic_shapes.ValueRanges(