[Feature Request] Add A3C

[tushdon2]

Motivation

Add A3C implementation as presented here so that the experiments of Rainbow can be added to the examples in further contributions.

Solution

Add A3C code like A2C and add its paper reproduction as an example.

Checklist

• [x] I have checked that there is no similar issue in the repo (required)

[tushdon2]

I am interested in working on this issue along with @shivank21.

[vmoens]

That would be wonderful!

[shivank21]

In A3C algorithm we are required to asynchronize the variables across multiple CPU threads. For that we were planning to use the threading library of python. Does that work here? Also it refers in the Documentation that "This module used to be optional, it is now always available." What does this really mean?

[vmoens]

Yes threading is built-in, so that should be ok!

[albertbou92]

Having A3C would be very cool!