Remove redundant ONNX samples and split requirements

Makefile

@@ -15,3 +15,6 @@ pylint-onnx:
 		nncf/experimental/onnx				\
 		nncf/quantization					\
 		tests/onnx examples/experimental/onnx
+
+test-install-onnx:
+	pytest tests/cross_fw/install/test_install.py --backend onnx

examples/experimental/onnx/__init__.py

@@ -0,0 +1,12 @@
+"""
+ Copyright (c) 2022 Intel Corporation
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+      http://www.apache.org/licenses/LICENSE-2.0
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+"""