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Minimize using Hessian-informed proximal gradient descent.

This function solves the regularized minimization problem

argmin{ Loss(x)
          + l1_regularizer * ||x||_1
          + l2_regularizer * ||x||_2**2
        : x in R^n }

where Loss is a convex C^2 function (typically, Loss is the negative log likelihood of a model and x is a vector of model coefficients). The Loss function does not need to be supplied directly, but this optimizer does need a way to compute the gradient and Hessian of the Loss function at a given value of x. The gradient and Hessian are often computationally expensive, and this optimizer calls them relatively few times compared with other algorithms.

grad_and_hessian_loss_fn callable that takes as input a (batch of) Tensor of the same shape and dtype as x_start and returns the triple (gradient_unregularized_loss, hessian_unregularized_loss_outer, hessian_unregularized_loss_middle) as defined in the argument spec of minimize_one_step.
x_start (Batch of) vector-shaped, float Tensor representing the initial value of the argument to the Loss function.
tolerance scalar, float Tensor representing the tolerance for each optimization step; see the tolerance argument of minimize_one_step.
l1_regularizer scalar, float Tensor representing the weight of the L1 regularization term (see equation above).
l2_regularizer scalar, float Tensor representing the weight of the L2 regularization term (see equation above). Default value: None (i.e., no L2 regularization).
maximum_iterations Python integer specifying the maximum number of iterations of the outer loop of the optimizer. After this many iterations of the outer loop, the algorithm will terminate even if the return value optimal_x has not converged. Default value: 1.
maximum_full_sweeps_per_iteration Python integer specifying the maximum number of sweeps allowed in each iteration of the outer loop of the optimizer. Passed as the maximum_full_sweeps argument to minimize_one_step. Default value: 1.
learning_rate scalar, float Tensor representing a multiplicative factor used to dampen the proximal gradient descent steps. Default value: None (i.e., factor is conceptually 1).
name Python string representing the name of the TensorFlow operation. The default name is "minimize".

x Tensor of the same shape and dtype as x_start, representing the (batches of) computed values of x which minimizes Loss(x).
is_converged scalar, bool Tensor indicating whether the minimization procedure converged within the specified number of iterations across all batches. Here convergence means that an iteration of the inner loop (minimize_one_step) returns True for its is_converged output value.
iter scalar, int Tensor indicating the actual number of iterations of the outer loop of the optimizer completed (i.e., number of calls to minimize_one_step before achieving convergence).


[1]: Jerome Friedman, Trevor Hastie and Rob Tibshirani. Regularization Paths for Generalized Linear Models via Coordinate Descent. Journal of Statistical Software, 33(1), 2010.

[2]: Guo-Xun Yuan, Chia-Hua Ho and Chih-Jen Lin. An Improved GLMNET for L1-regularized Logistic Regression. Journal of Machine Learning Research, 13, 2012.