R counterpart of CVXPY's Atom._grad(self, values). Returns a list
of one Jacobian per argument, shape (prod(arg.shape), prod(self.shape)).
Called by the chain-rule walker grad(x). The leading dot follows R's
private-name convention (cf. .Machine, .libPaths, .cvxr_*); the
name maps one-to-one onto CVXPY's _grad.
.grad(x, values, ...)A list of sparse Jacobians (one per argument).
An atom expression.
A list of numeric values, one per argument.
Reserved for future use; method dispatch ignores it.