Extend GenericDict support to integer keys.

[mlej8]

hi, I noticed converting HashMap with i64 as keys to tch::IValue::GenericDict results in an object of type Dict[Any,Any] on the python side:

pub fn create_int_tensor_dict_ivalue(hm: &HashMap<i64, tch::Tensor>) -> tch::IValue {
    tch::IValue::GenericDict(
        hm.iter()
            .map(|(k, v)| {
                (
                    tch::IValue::Int(*k),
                    tch::IValue::Tensor(tch::Tensor::shallow_clone(v)),
                )
            })
            .collect::<Vec<(tch::IValue, tch::IValue)>>(),
    )
}

However, hashmaps with strings as keys are typed as Dict[str, Tensor], could we add support for int as keys ?