Based on the stacktrace I would guess that this inplace manipulation is disallowed:
self.feature_volume[inds_0, inds_1, inds_2, :] = self.fusion_learner(torch.cat(inputs_i), self.feature_volume[inds_0, inds_1, inds_2, :])

so try to replace it with assigning the result to a new tensor instead.