nn.CrossEntropy expects a model output of the shape [batch_size, nb_classes, *additional_dimensions] and a target in [batch_size, *additional_dimensions] containing the class indices in the range [0, nb_classes] as explained in the docs 11.
For an output of [batch_size=64, nb_classes=124, additional=6], the target should have the shape [64, 6] and contain values in [0, 123].
Could you explain your use case a bit and what the dimensions in your tensors mean?