cuTENSOR: Fix reference to undefined variable

[lkdvos]

This is a small fix about a typo in figuring out the compute_type in the plan_reduction function, having three arguments, while only two tensors are involved.

On a different note, this is never caught in the tests, as reduce! actually determines the compute_type before it creates the plan, and there it is correctly implemented. (This is also true for contract!, permute!, elementwise_binary_exectute! and elementwise_trinary_execute!, but as far as I can tell, these are all correct). I think it could make sense to remove this duplicate code, and just let the plan_**operation** functions be the only place where the compute_type is determined.
I can file a PR for that as well, or update this one, but this is a bit larger of a change so I'd rather get some feedback first.

[maleadt]

Thanks for the fix!

I think it could make sense to remove this duplicate code, and just let the plan_**operation** functions be the only place where the compute_type is determined.

Makes sense. Let's merge the immediate fix now, and I'd be happy to take a PR refactoring the remainder 🙂