Update test to handle NaN compare after NLL change
Pytorch nll behavior has changed - now, where the sum of the weight
values is zero, the output will be NaN. Our LTC tests must be updated
accordingly.
- pytorch/pytorch changed NLL behavior
(https://github.com/pytorch/pytorch/pull/64572)
- pytorch/xla introduced a corresponding change to test utils
(https://github.com/pytorch/xla/pull/3144)
