Improvements to sampling based differentiation

[mhmukadam]

• Broadcast retract_and_update_variables to improve efficiency of sampling procedure in LEO. See original comment.
• Move get_average_sample_cost to somewhere commonly accessible: utils or theseus layer. See original comment.
• Reuse the linear solve solution (delta) from theseus layer forward to avoid recomputing in the sampling function. See original comment.