Add an adaptive optimizer

[soranjh]

Context:
This PR adds an adaptive optimizer.

Description of the Change:
The AdaptiveOptimizer class is added to qml.optimize.

Benefits:
The new functionality makes it easy for the users to build quantum chemistry circuits adaptively.

Possible Drawbacks:

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.

[codecov]

Codecov Report

Merging #3192 (a7a92af) into master (65e335e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3192   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         277      278    +1     
  Lines       24578    24621   +43     
=======================================
+ Hits        24505    24548   +43     
  Misses         73       73           

Impacted Files	Coverage Δ
pennylane/optimize/__init__.py	100.00% <100.00%> (ø)
pennylane/optimize/adaptive.py	100.00% <100.00%> (ø)

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[ixfoduap]

This optimizer is awesome!

I'm making mostly small suggestions, except perhaps the addition of a few more tests

[obliviateandsurrender]

Looking good! Just leaving a couple of suggestions.