Optimize scatter_add/scatter_reduce in BFloat16/Half data type in CPU backend (#103427)
### Description
This PR is to optimize scatter_add/scatter_reduce of BFloat16/Half data type in CPU backend, which is one task in https://github.com/pyg-team/pytorch_geometric/issues/7057. Main point is creating a buffer among threads to accumulate intermediate data as fp32 data type.
Next step:
- [x] Add benchmarks
- [x] Extend to Half
- [x] Simplify code
### Performance test (Updated)
Test BFloat16 in Intel(R) Xeon(R) Platinum 8380 CPU @ 2.30GHz
With jemalloc and iomp
Single socket (40C)
Single core
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103427
Approved by: https://github.com/mingfeima, https://github.com/albanD
