Fix missing import in tutorial (#2065)

Summary:
Pull Request resolved: #2065

Looks like land checks ignored OSS failure

Reviewed By: Balandat

Differential Revision: D50524118

fbshipit-source-id: 71be92193becb49cd3ec3670333654c1d63d3f81

Author: saitcakmak

tutorials/custom_botorch_model_in_ax.ipynb

@@ -88,13 +88,16 @@
       },
       "outputs": [],
       "source": [
+        "from typing import Optional\n",
+        "\n",
         "from botorch.models.gpytorch import GPyTorchModel\n",
         "from botorch.utils.datasets import SupervisedDataset\n",
         "from gpytorch.distributions import MultivariateNormal\n",
         "from gpytorch.kernels import RBFKernel, ScaleKernel\n",
         "from gpytorch.likelihoods import GaussianLikelihood\n",
         "from gpytorch.means import ConstantMean\n",
         "from gpytorch.models import ExactGP\n",
+        "from torch import Tensor\n",
         "\n",
         "\n",
         "class SimpleCustomGP(ExactGP, GPyTorchModel):\n",