Explicit `dtype` for the node and edge attrs?

[yfukai]

Currently, the dtype of node and edge attributes is inferred from the default value provided. It could be helpful to specify an explicit dtype to manage situations where the default value's type may differ from the target object's type (for example, None versus Mask).

[yfukai]

... or we can enforce the default value to be the same dtype as the attr values themselves.