Add missing __init__ file (#1672)

Summary:
~Although I can't reproduce the error locally,~ I believe this is the cause of the `ModuleNotFoundError: No module named 'ax.models.torch.botorch_modular.input_constructors'` errors in https://github.com/pytorch/botorch/actions/runs/5329180791/jobs/9654605153

Actually, I managed to repro. This is indeed the cause / fix for those failures.

Before:
<img width="851" alt="Screenshot 2023-06-20 at 7 26 59 PM" src="https://github.com/facebook/Ax/assets/9263852/134ffec8-8335-4dc3-8e4d-77c4c50adc9f">
After:
<img width="849" alt="Screenshot 2023-06-20 at 7 29 56 PM" src="https://github.com/facebook/Ax/assets/9263852/e762caf8-4abc-4c80-bab4-2d7cad8d83ff">

Pull Request resolved: #1672

Reviewed By: Balandat

Differential Revision: D46884977

Pulled By: saitcakmak

fbshipit-source-id: ef733027ca5a5e27765103492e7be3a178829e18

ax/models/torch/botorch_modular/input_constructors/__init__.py

@@ -0,0 +1,5 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.

sphinx/source/models.rst

@@ -272,6 +272,14 @@ ax.models.torch.botorch_modular.kernels module
     :undoc-members:
     :show-inheritance:
 
+ax.models.torch.botorch_modular.input_constructors.covar_modules module
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. automodule:: ax.models.torch.botorch_modular.input_constructors.covar_modules
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
 ax.models.torch.cbo_lcea module
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~