Reduce src into input along dim at positions given by index.

Multi-reduction sibling of scatter_add. When several entries of src target the same position the chosen reduce op decides how they combine. include_self controls whether the existing value in input participates in the reduction or is replaced.

Every reduction treats a position no index entry names the same way: it keeps input's value, and the gradient reaches input there. With include_self=False a position index does name takes nothing from input — neither its value nor a gradient. For 'amax' / 'amin' the gradient of a position splits evenly among the values tied for its result; with include_self=False input's value is not among them even when it equals the result, so the shares still add up to the incoming gradient. This differs from the reference framework, which counts that value as a tie and then drops its share.