thrust::tuple_size fails for "const thrust::pair<T1,T2>"

[jrhemstad]

As seen in this example, thrust::tuple_size< const thrust::tuple<T1, T2>> will fail to compile:

https://godbolt.org/z/M177q6Ye6

This is because tuple_size has a partial specialization for pair<T1,T2>, but not const pair<T1, T2>.

As a result, tuple_size for a const pair will hit the generic template for tuple_size, which is only valid for thrust::tuple types.

[jrhemstad]

Same problem with tuple_element: https://godbolt.org/z/37Wj6Wene