[inductor] Refactor common triton imports into one function (#121438)
This means when codegen depends on a particular import we only need to
add it in one place and it's applied to all triton kernels.
This also changes codegen slightly so instead of generating
`@pointwise` we now generate `@triton_heuristics.pointwise` just so
the imports are the same for all kernel types.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121438
Approved by: https://github.com/lezcano
