defines thrust::tuple_element consistently with std::tuple_element

[andrewcorrigan]

I propose changing the template arguments of thrust::tuple_element to match std::tuple_element:

template< std::size_t I, class T >
class tuple_element;

This is consistent with the implementation used by <cuda/std/tuple>

While I understand that thrust::tuple_element will likely be replaced alongside thrust::tuple pending resolution of NVIDIA/cccl#742, the motivation for this PR is that the overload of thrust::tuple_element for thrust::pair will also need to be replaced, since it still uses template<int, typename>. This PR enables branches such as my variadic tuple branch to BJHVT (bring-jared-hoberock's-variadic-tuple) while we await mainline variadic thrust::tuple support.

With -DTHRUST_DEVICE_SYSTEM=CPP:

100% tests passed, 0 tests failed out of 151

Total Test time (real) = 109.05 sec

[alliepiper]

LGTM. We'll have to change this anyway for NVIDIA/cccl#742, might as well get it out of the way now.

[alliepiper]

DVS CL 29194450.