Fix ref / decomposition index_add (#86266)
The decomposition of `index_add` was using `slice(None)`, when it should
use just `None`.
The reference for index_add was also wrong, as `x[idx] += t` does not
use atomic add, so it does not work when several `idx`s point to the
same location.
This PR adds extra reference inputs to help test for this.
Fixes https://github.com/pytorch/torchdynamo/issues/1356
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86266
Approved by: https://github.com/ngimel
