[Inductor/cpp] Fix reduction on pre clang-10 (#103347)
`#pragma omp declare reduction` is not supported before clang-10 and results in a misleading compiler error in the following example:
```c++
template<typename T>
T max_propagate_nan(T, T);
extern "C" void cpp_fused_argmax_max_sum_0(const float* in_ptr0,
float* out_ptr0,
float* out_ptr1,
long* out_ptr2)
{
float tmp_acc0 = 0;
float tmp_acc1 = -std::numeric_limits<float>::infinity();
float tmp_acc2 = std::numeric_limits<float>::infinity();
struct IndexValue_7 {size_t index; float value;};
IndexValue_7 tmp_acc3{0, -std::numeric_limits<float>::infinity()};
#pragma omp declare reduction(argmax : IndexValue_7 : omp_out.value = omp_in.value < omp_out.value ? omp_out.value : omp_in.value, omp_out.index = omp_in.value < omp_out.value ? omp_out.index : omp_in.index) initializer(omp_priv = {0, -std::numeric_limits<float>::infinity()})
for(long i0=static_cast<long>(0L); i0<static_cast<long>(3L); i0+=static_cast<long>(1L))
{
auto tmp0 = in_ptr0[static_cast<long>(i0)];
tmp_acc0 = tmp_acc0 + tmp0;
tmp_acc1 = max_propagate_nan(tmp_acc1, tmp0);
if (tmp_acc3.value < tmp0) {
tmp_acc3.index = i0; tmp_acc3.value = tmp0;
}
}
out_ptr0[static_cast<long>(0L)] = tmp_acc0;
out_ptr1[static_cast<long>(0L)] = tmp_acc1;
out_ptr2[static_cast<long>(0L)] = tmp_acc3.index;
}
```
```
% clang++-10 -std=c++17 -fopenmp bar.cpp -c -O3
% clang++-9 -std=c++17 -fopenmp bar.cpp -c -O3
bar.cpp:17:149: error: expected ')'
#pragma omp declare reduction(argmax : IndexValue_7 : omp_out.value = omp_in.value < omp_out.value ? omp_out.value : omp_in.value, omp_out.index = omp_in.value < omp_out.value ? omp_out.index : omp_in.index) initializer(omp_priv = {0, -std::numeric_limits<float>::infinity()})
^
bar.cpp:17:34: note: to match this '('
#pragma omp declare reduction(argmax : IndexValue_7 : omp_out.value = omp_in.value < omp_out.value ? omp_out.value : omp_in.value, omp_out.index = omp_in.value < omp_out.value ? omp_out.index : omp_in.index) initializer(omp_priv = {0, -std::numeric_limits<float>::infinity()})
^
1 error generated.
```
Also, remove unnecessary `struct` keyword in front of type, as C++ compiler already assumes that (and again, it causes problem with clang++-10 implementation)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103347
Approved by: https://github.com/voznesenskym
