Improve discussion of derivatives

[sandwichmaker]

List of things to add:

• Why does the quality of derivatives matter?
• Discuss, forward v/s backward automatic differentiation and relation to backprop, impact of large parameter block sizes on differentiation performance.
• Inverse function theorem
• Calling iterative routines.
• Reference to how numeric derivatives lead to slower convergence.
• Pitfalls of Numeric differentiation.
• Ill conditioning of numeric differentiation/dependence on curvature.