[Nano] Remove redundant exposed apis and add basic openvino unit tests

[zhentaocc]

Description

This PR is to fix some unresolved issues in #5115

1. As mentioned in [Nano]Make OpenVINOModel as public API #5115, we should expose bigdl.nano.openvino.OpenVINOModel only for basic openvino usage.
2. Add basic unit tests w\o pytorch/tf installed.

1. Why the change?

1. expose Pytorch/TF APIs in bigdl.nano.openvino is not that reasonable.
2. We should have UTs to make sure openvino work fine w\o pytorch/tf.

2. User API changes

N/A

3. Summary of the change

1. Only expose OpenVINOModel in bigdl.nano.openvino
2. Add UTs for OpenVINOModel usage

4. How to test?

• Unit test

[zhentaocc]

http://10.112.231.51:18888/job/BigDL-NanoTests-PR-Validation/324/
http://10.112.231.51:18888/job/BigDL-PRVN-chronos-Python-Spark-2.4-py37-ray-part2/1195/

[zhentaocc]

http://10.112.231.51:18888/job/BigDL-NanoTests-PR-Validation/324/
http://10.112.231.51:18888/job/BigDL-PRVN-chronos-Python-Spark-2.4-py37-ray-part2/1195/