Add warning for reverse mode routines

[joanibal]

As it is, master_b doesn't do a forward pass so some other routine that sets the variables must be called first. Otherwise, computeJacobianVectorProductBwd() will return garbage.

We should add a warning (on the python layer?) to make sure folks know this so they don't waste their time like me.

[joanibal]

I don't have the authority to assign myself, but I'll do it sometime. Just wanted to document it here so I don't forget.

[anilyil]

Is this fixed? Is there a PR for this? I am not sure if we can come up with a general case that covers all bases for this. master_b can be called in a range of scenarios, and I think its up to the user to make sure that all fwd values are set correctly. However, we can document it and raise warnings on the python level at least.