Among our different back-end implementations the type used to accumulate the intermediate values of a scan differs.
This result is inconsistent scan results between different back-ends when using input and output containers with different value types.
For example scanning over an array of 16-bit int inputs and storing into a array of 32-bit int outputs could lead to truncation of the accumulated values to 16-bits despite the output being able to store 32-bits.
The behavior should be made to consistently use the output type for the accumulation type.

We default out initial values to the type from the input container.
https://github.com/LLNL/RAJA/blob/develop/include/RAJA/pattern/scan.hpp#L262
We use the type of the output container to accumulate in the sequential backend.
https://github.com/LLNL/RAJA/blob/develop/include/RAJA/policy/sequential/scan.hpp#L146
Cub uses the type of the output container to accumulate for the cuda backend.
https://github.com/dmlc/cub/blob/05eb57faa0a4cac37c2a86fdf4b4dc865a95a1a3/cub/agent/agent_scan.cuh#L110
Rocprim uses the type of the input container to accumulate for the hip backend.
https://github.com/ROCm/rocPRIM/blob/develop/rocprim/include/rocprim/device/device_scan.hpp#L653