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.

python/triton/backends/compiler.py

@@ -210,6 +210,9 @@ def get_property_key(val, align):
             return "1"
         return "N"
 
+    def __repr__(self):
+        return f"AttrsDescriptor.from_dict({self.to_dict()!r})"
+
 
 @dataclass(frozen=True)
 class GPUTarget(object):