autodiff for matrix add and subtract

Description

There are lots of functions that don't have reverse mode specializations.

Matrix operations

All of these would be very useful, espcially the basic arithmetic of the first five and inverse_spd because it's so important.

• add
• subtract
• minus
• elt_multiply
• elt_divide
• diag_post_multiply
• diag_pre_multiply
• quad_form_diag
• chol2inv
• `inverse_spd

All of the constraints would be good to specialize. These get used everywhere (the least important are corr_constrain and cov_matrix_constrain---the cholesky forms are more important for our recommended usages).

• lb_constrain
• ub_constrain
• lub_constrain
• corr_constrain
• cov_matrix_constrain
• cholesky_factor_constrain
• cholesky_corr_constrain
• offset_multiplier_constrain

Then there are some reductions:

• mean
• sd
• variance

There are probably more.

Feel free to solve one or more of these in a PR.

Current Version:

v3.1.0

Start by reading the existing reverse-mode specializations for the listed matrix operations, especially add and subtract, and compare them with the current implementations of those operations. Choose a specific operation and define done as adding its reverse-mode specialization with coverage for the relevant behavior; the issue does not name files or tests to run.