[inductor][cpp] fix mul for uint8 (#98473)
Fixes #98149
The type of `mul`'s output is not inconsistent with its input. This PR fixes the type of `mul`'s output.
Here is the output code for the newly added test case `pow+cos`. `tmp4` is 1024 before fixing and 0 after fixing.
#### Before fixing
```
auto tmp0 = in_ptr0[static_cast<long>(0)]; // tmp0 is unsigned_char
auto tmp1 = tmp0 * tmp0; // tmp1 is int
auto tmp2 = tmp1 * tmp1; // tmp2 is int
auto tmp3 = tmp2 * tmp0; // tmp3 is int
auto tmp4 = static_cast<float>(tmp3); // tmp4 is float
auto tmp5 = std::cos(tmp4);
out_ptr0[static_cast<long>(0)] = tmp5;
```
#### After fixing
```
auto tmp0 = in_ptr0[static_cast<long>(0)]; // tmp0 is unsigned_char
auto tmp1 = decltype(tmp0)(tmp0 * tmp0); // tmp1 is unsigned_char
auto tmp2 = decltype(tmp1)(tmp1 * tmp1); // tmp2 is unsigned_char
auto tmp3 = decltype(tmp2)(tmp2 * tmp0); // tmp3 is unsigned_char
auto tmp4 = static_cast<float>(tmp3); // tmp4 is float
auto tmp5 = std::cos(tmp4);
out_ptr0[static_cast<long>(0)] = tmp5;
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98473
Approved by: https://github.com/EikanWang, https://github.com/jgong5, https://github.com/jansel
