[Exploration] Removing autograd from list of mandatory dependencies

[albi3ro]

Context:

We are wanting to downgrade the priority given autograd, given the package is no longer maintained and has fewer features than the other interfaces. One way to do this is by making autograd an optional dependency, that must be installed by the user.

Description of the Change:

This prototype creates a version of pennylane where autograd is no longer a mandatory dependency. The version right now is rather rough and clunky, but we are able to use pennylane without autograd being installed.

Benefits:

Possible Drawbacks:

Making the CI work might be a nightmare. Actually, it will very likely be nightmare.

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.