Relax input type requirements for thrust::merge

`thrust::merge` states this requirement:

> InputIterator2 and InputIterator1 have the same value_type

https://thrust.github.io/doc/group__merging_ga3d2776685a00dca265399f411784acec.html

cppreference for `std::merge` has a slightly more relaxed requirement:

> The signature of the comparison function should be equivalent to the following:
> bool cmp(const Type1 &a, const Type2 &b);
> The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​

https://en.cppreference.com/w/cpp/algorithm/merge

However, I can't find this requirement anywhere in the standard, so I don't know where `cppreference` is getting that from
* Current https://eel.is/c++draft/alg.merge
* C++17 https://timsong-cpp.github.io/cppwp/n4659/alg.merge

Furthermore, none of gcc/clang/msvc seem to actually require this:
https://godbolt.org/z/v1WcW5scv

The Thrust requirement stems from the fact that it loads elements from both input ranges into a single thread local, register array, thus requiring them to be the same type: https://github.com/NVIDIA/thrust/blob/7afdb87ad0e0a213a6f7e50548cbabc7e19f400f/thrust/system/cuda/detail/merge.h#L480-L485

A cursory look through the algorithm makes it seem like it wouldn't be too difficult to instead use two distinct storage arrays for each input range.