Move HSS kernel GP and utils into OSS BoTorch (#3284)

Summary:
Pull Request resolved: #3284

Move the hierarchical search space (HSS) conditional kernel GP and helper
utilities from `botorch_fb.models.hss` into open-source BoTorch at
`botorch.models.hierarchical`.

Moves (history preserved via `sl mv`):
- `botorch_fb/models/hss/hierarchical_conditional_kernel_gp.py` →
  `botorch/models/hierarchical/conditional_kernel_gp.py`
- `botorch_fb/models/hss/utils.py` →
  `botorch/models/hierarchical/utils.py`
- Corresponding tests under
  `botorch_fb/test/models/hss/` → `botorch/test/models/hierarchical/`.

New `__init__.py` at `botorch/models/hierarchical/` exports
`HierarchicalConditionalKernel`, `HierarchicalConditionalKernelGP`, and
`HierarchicalConditionalKernelMultiTaskGP`.

License headers swapped to the OSS MIT license header.

Reviewed By: ItsMrLin

Differential Revision: D101814586

fbshipit-source-id: 65bdb71f9e3860f977868b8f78f332fde965b194

Author: saitcakmak

botorch/models/hierarchical/__init__.py

@@ -0,0 +1,18 @@
+#!/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.
+
+from botorch.models.hierarchical.conditional_kernel_gp import (
+    HierarchicalConditionalKernel,
+    HierarchicalConditionalKernelGP,
+    HierarchicalConditionalKernelMultiTaskGP,
+)
+
+
+__all__ = [
+    "HierarchicalConditionalKernel",
+    "HierarchicalConditionalKernelGP",
+    "HierarchicalConditionalKernelMultiTaskGP",
+]