Add string representation for AttrsDescriptor (#4888)

The string representation allows PyTorch Inductor to
serialize/derserialize the `AttrsDescriptor` to the `@triton.heuristics`
block in the generated code.

Author: alexbaden

python/triton/backends/compiler.py

@@ -211,7 +211,7 @@ def get_property_key(val, align):
         return "N"
 
     def __repr__(self):
-        return f"AttrsDescriptor.from_dict({self.arg_properties})"
+        return f"AttrsDescriptor.from_dict({self.to_dict()!r})"
 
 
 @dataclass(frozen=True)