Feat: added method get_distribution for class ActorCriticPolicy

[cyprienc]

Description

New method get_distribution for class ActorCriticPolicy returning current action distribution given observations

Motivation and Context

This is related to Stable-Baselines-Team/stable-baselines3-contrib#40 (comment)
TRPO will require the Distribution of the current policy.

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 reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

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[araffin]

Suggested change

	- Added method `get_distribution` for `ActorCriticPolicy` (@cyprienc)
	- Added method `get_distribution` for ``ActorCriticPolicy`` (@cyprienc)

[araffin]

Only a test is missing, otherwise LGTM ;)

[araffin]

I added the test and fixed the changelog in #566
but I couldn't push the changes to your PR because you were using master branch... so I opened another PR.

[cyprienc]

Ok - thanks a lot - I am closing this one then