[inductor] Create triton_helpers module for helper functions (#99880)
This changes codegen of `torch.prod` from:
```python
tl.reduce(tmp2, 1, _prod_accumulate)[:, None]
```
where `_prod_accumulate` is defined elsewhere, to
```python
triton_helpers.prod(tmp2, 1)[:, None]
```
A quirk I uncovered though is that `TritonCodeCache` breaks if you
define any new symbol beginning with `triton_`, since it assumes that
must be the kernel name. Instead, I've made the kernel name an
explicit argument to `async_compile.triton` so it doesn't have to guess.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99880
Approved by: https://github.com/ngimel
