[BugFix] Pickable buffer

[albertbou92]

Description

This PR makes the ReplayBuffer pickable. Also a pickable test for the replay buffers.

Tries to solve the following issue: #1408

Motivation and Context

It is convenient to have ReplayBuffer and other classes pickable for features like distributed training, were sometimes need to be serialised.

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

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

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

Checklist

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• I have read the CONTRIBUTION guide (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.

[albertbou92]

Should the test be more strict? we can not check that each original dict object is equal (==) to its deserialised version because most classes don't have the eq() method overwritten.

[vmoens]

Nope that unit test covers what we want to fix

[vmoens]

LGTM