[FEA] Add cuFFT helpers

Spawned from this cuML PR comment: https://github.com/rapidsai/cuml/pull/3058#discussion_r518439663

That PR adds the below code, which would be useful in raft.

#include <cufft.h>
#include <raft/error.hpp>

namespace raft {

/**
 * @brief Exception thrown when a cuFFT error is encountered.
 */
struct cufft_error : public raft::exception {
  explicit cufft_error(char const* const message) : raft::exception(message) {}
  explicit cufft_error(std::string const& message) : raft::exception(message) {}
};

const char* getCufftErrStr(cufftResult status) {
  // https://docs.nvidia.com/cuda/cufft/index.html#cufftresult
  switch (status) {
    case CUFFT_SUCCESS:
      return "The cuFFT operation was successful.";
    case CUFFT_INVALID_PLAN:
      return "cuFFT was passed an invalid plan handle.";
    case CUFFT_ALLOC_FAILED:
      return "cuFFT failed to allocate GPU or CPU memory.";
    case CUFFT_INVALID_VALUE:
      return "User specified an invalid pointer or parameter.";
    case CUFFT_INTERNAL_ERROR:
      return "Driver or internal cuFFT library error.";
    case CUFFT_EXEC_FAILED:
      return "Failed to execute an FFT on the GPU.";
    case CUFFT_SETUP_FAILED:
      return "The cuFFT library failed to initialize.";
    case CUFFT_INVALID_SIZE:
      return "User specified an invalid transform size.";
    case CUFFT_INCOMPLETE_PARAMETER_LIST:
      return "Missing parameters in call.";
    case CUFFT_INVALID_DEVICE:
      return "Execution of a plan was on different GPU than plan creation.";
    case CUFFT_PARSE_ERROR:
      return "Internal plan database error.";
    case CUFFT_NO_WORKSPACE:
      return "No workspace has been provided prior to plan execution.";
    case CUFFT_NOT_IMPLEMENTED:
      return "Function does not implement functionality for parameters given.";
    case CUFFT_NOT_SUPPORTED:
      return "Operation is not supported for parameters given.";
    default:
      return "Unknown error.";
  }
}

/**
 * @brief Error checking macro for cuFFT functions.
 *
 * Invokes a cuFFT function. If the call does not return CUFFT_SUCCESS, throws
 * an exception detailing the error that occurred.
 */
#define CUFFT_TRY(call)                                                     \
  do {                                                                      \
    const cufftResult status = call;                                        \
    if (status != CUFFT_SUCCESS) {                                          \
      std::string msg{};                                                    \
      SET_ERROR_MSG(msg,                                                    \
                    "cuFFT error encountered at: ", "call='%s', Reason=%s", \
                    #call, raft::getCufftErrStr(status));                   \
      throw raft::cufft_error(msg);                                         \
    }                                                                       \
  } while (0)

class CuFFTHandle {
 public:
  CuFFTHandle() { CUFFT_TRY(cufftCreate(&handle)); }
  ~CuFFTHandle() { cufftDestroy(handle); }
  operator cufftHandle() const { return handle; }

 private:
  cufftHandle handle;
};

}  // namespace raft

Start by reviewing the cuML PR comment referenced in the issue, then inspect how RAFT currently handles errors and resource-owning helpers. The issue's example names cufft.h and raft/error.hpp; done means the proposed cuFFT error handling and CuFFTHandle helper are integrated into RAFT consistently.