feat: concat kernel for Alpaka

[Saransh-cpp]

Adds concat kernel for the alpaka backend with the following signature:

template <typename TAcc, typename T>
ALPAKA_FN_ACC void operator()(TAcc const& acc, T const* const* input_ptrs, T* output,
                              std::size_t const* const* input_strides_ptrs, std::size_t const* axis_sizes,
                              std::size_t num_inputs, std::size_t axis, std::size_t const* output_strides,
                              std::size_t const* output_shape) const

Standalone kernel - https://github.com/Saransh-cpp/SOFIE-ALPAKA/blob/main/kernels/concat.hpp - with a minimal test - https://github.com/Saransh-cpp/SOFIE-ALPAKA/blob/main/tests/test_concat.cpp.