fix: resolve std::optional vs c10::optional namespace conflict

[atakan-topaloglu]

Summary

GCC (8.5.0/10.3.0) pulls in <optional> via <algorithm>, exposing std::optional and std::nullopt.
torch-points-kernels also includes PyTorch headers, which expose c10::optional and c10::nullopt.
Because neighbors.h had using namespace std;, both namespaces leaked, causing ambiguity errors.

Root Cause

• cpu/include/neighbors.h used using namespace std;.
• <algorithm> transitively includes <optional>.
• Both std::optional and c10::optional become visible, and this leads to ambiguous references.

Fix

• Remove using namespace std; from neighbors.h.
• Change vector<> to std::vector<> in header declarations.
• In neighbors.cpp, add targeted imports:
  • using std::vector; using std::pair; using std::max;
• Update knn.cpp to use std::vector<> explicitly.

Verification

Build succeeds on:

• GCC 10.3.0 and GCC 8.5.0, CUDA 11.8, PyTorch 2.x