Fix GPU segfaults in PW USPP calculations

[chengleizheng]

Summary

Fix GPU segfaults in PW USPP calculations caused by CPU code accessing GPU
memory pointers. Added CPU intermediate buffers and D2H/H2D sync patterns
in three USPP-specific code paths.

Changes

• hamilt_pw: sPsi reads qq_nt from CPU backup (qq_nt.ptr) instead of
  GPU pointer, fills a CPU buffer, then syncs to GPU for cuBLAS.

• elecstate_pw (cal_becsum): becp computed on GPU via gemm then synced
  to CPU; auxk1/auxk2 use CPU vectors; aux_gk computed on GPU then synced
  to CPU; becsum allocated on CPU (consumed by CPU-only force/stress code).

• elecstate_pw (add_usrho): Added D2H/H2D wrappers around real2recip
  and recip2real FFT calls, which read/write via CPU loops.

• elecstate_pw (addusdens_g): Rewritten to use pure CPU computation
  (CPU BLAS zgemm_, CPU radial_fft_q); ylmk0 computed on GPU with H2D'd
  gcar then D2H'd back.

Test plan

• Run PW USPP test cases (003, 008, 009) on GPU, verify no segfault
• Run compute-sanitizer to confirm 0 GPU memory errors