updated policies.py to support tf 2.x

[PsVenom]

replaced tf 1.0 functions with tf.compat.v1 + replaced tf.contrib with tf-slim

Description

replaced tf 1.0 functions with tf.compat.v1 + replaced tf.contrib with tf-slim. This changes allows the following tutorial to work on tensorflow 2.0 -> https://stable-baselines.readthedocs.io/en/master/guide/pretrain.html#pretrain

Motivation and Context

the library is made for tf 1.x, which has been outdated for years now

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have ensured pytest and pytype both pass (by running make pytest and make type).