Add support for tensorflow 2.x

[LittleD3092]

Description

This pull request simply contains the following changes:

• Fixed import issues for tensorflow 2.x.
  • There are other pull requests (tensorflow compatibility with TF 2 #1216, TensorFlow 2.0 above support added #1098 ) that does the same thing. However, I decided to use try-except cases to keep tensorflow 1.x support.
  • While using tensorflow 2.x, a python module tf_slim is required. This pull request does not contain the installation instructions because I think TensorFlow 2.0 above support added #1098 has better documentations.
• cv2.namedWindow('', 0) will trigger an assertion error for not having a window name. Therefore I editted it to cv2.namedWindow('camera window', 0).

Tests

It has been tested on tensorflow 2.8.0, but not tested on tensorflow 1.x.

My testing environment is as follows:

• Ubuntu 20.04.5 LTS
• Python 3.8.10
• Tensorflow version: tf.__version__ is '2.8.0'.
• Opencv version: cv2.__version__ is '4.6.0'.