code: def test_error_on_fidelity_only(self):
      x1 = torch.tensor([[0.1], [0.3]])
      x2 = torch.tensor([[0.5], [0.7]])
      kernel = LinearTruncatedFidelityKernel(
          fidelity_dims=[0], dimension=1, nu=2.5
      )
      with self.assertRaises(RuntimeError):
          kernel(x1, x2).to_dense()
  
imports: import itertools
  import torch
  from botorch.exceptions import UnsupportedError
  from botorch.models.kernels.linear_truncated_fidelity import LinearTruncatedFidelityKernel
  from botorch.utils.testing import BotorchTestCase
  from gpytorch.kernels.matern_kernel import MaternKernel
  from gpytorch.kernels.rbf_kernel import RBFKernel
  from gpytorch.priors.torch_priors import GammaPrior, NormalPrior
  from gpytorch.test.base_kernel_test_case import BaseKernelTestCase
label_desc: Use image node_id 14 for calling the TestLinearTruncatedFidelityKernel obj's underlying member method code with example usage: obj.test_error_on_fidelity_only() without return types