perf(models/FsmnVADStreaming): optimize GetFrameState and PopDataToOutputBuf (#2177)

- In GetFrameState(), pass generator to sum() instead of generating a list, ~10% gain in a 21s sample
- In GetFrameState(), cast `sum_score` (a tensor) to float to reduce calling to tensor lib,
  ~13% gain in a 23s example
- In PopDataToOutputBuf(), remove unused `out_pos` and related calculation, ~10% gain in a 27s sample