Make CSPDarknet a Functional subclass model and remove closures

[ianstenbit]

This should follow the example of #1412 and #1401, including

• Docstring updates
• Colab demonstrating that weight loading works (if pre-trained weights exist) and numerics are identical

[soma2000-lang]

working on this.https://github.com/keras-team/keras-cv/pull/1478

[YimjuneLiu]

Hi @ianstenbit @soma2000-lang , Would you mind if I joined the job?

[soma2000-lang]

@YimjuneLiu I am already working on this,will open a pr soon .Can you do some other task?

[YimjuneLiu]

Sure.