Re-enable some C++ warnings (#142332)

It enables some C++ warnings since the code base is fairly clean. Meanwhile, Wextra-semi is disabled on CUDA generated code since there is no way to fix them without the cooperation of CUDA team.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/142332
Approved by: https://github.com/albanD, https://github.com/eqy

functorch/csrc/dim/dim.cpp

@@ -1888,7 +1888,6 @@ static PyObject* order(PyObject *_,
         }
     }
 
-    int ndim = 0;
     int insert_point = -1;
     Slice<DimEntry> new_levels;
     for (auto l : levels) {
@@ -1896,7 +1895,6 @@ static PyObject* order(PyObject *_,
             continue;
         }
         if (l.is_positional()) {
-            ndim++;
             if (insert_point == -1) {
                 insert_point = new_levels.size();
                 new_levels.extend(A, flat_positional_dims);