Using observation space dimension size for the Actor and Critic models.

Hi, @philtabor !

Thanks for the awesome code and video!
Those really help me to study and understand reinforcement learning.

 I found that the actor and critic model are not using observation space dim as their `input_dim`.
Shouldn't it be the same dim size as observation space?

What do you think?

And I made a pull request #39 about above.
I really appreciate your review!