Fix a logging bug in TrustRegionMinimizer.

Upon encountering an unsuccessful step (one where the cost goes up)
the the trust region minimizer failed to populate the gradient norm
in the IterationSummary. This would cause the gradient norm to be
logged as zero which is incorrect. Instead it should be the gradient
norm at the current point.

This CL fixes this issue.

Before:
iter      cost      cost_change  |gradient|   |step|    tr_ratio  tr_radius  ls_iter  iter_time  total_time
   0  1.115206e+07    0.00e+00    1.90e+07   0.00e+00   0.00e+00  1.00e+04        0    2.72e-01    1.33e+00
   1  3.687552e+06    7.46e+06    1.84e+08   2.86e+03   6.91e-01  1.06e+04        1    1.32e+00    2.65e+00
   2  3.670266e+10   -3.67e+10    0.00e+00   3.27e+03  -1.07e+04  5.30e+03        1    7.52e-01    3.40e+00
   3  4.335397e+07   -3.97e+07    0.00e+00   2.74e+03  -1.16e+01  1.32e+03        1    7.28e-01    4.13e+00
   4  1.345488e+06    2.34e+06    4.12e+07   1.55e+03   6.87e-01  1.40e+03        1    9.31e-01    5.06e+00
   5  5.376653e+05    8.08e+05    9.99e+06   6.64e+02   7.46e-01  1.59e+03        1    9.64e-01    6.03e+00

After:
iter      cost      cost_change  |gradient|   |step|    tr_ratio  tr_radius  ls_iter  iter_time  total_time
   0  1.115206e+07    0.00e+00    1.90e+07   0.00e+00   0.00e+00  1.00e+04        0    2.37e-01    1.13e+00
   1  3.687552e+06    7.46e+06    1.84e+08   2.86e+03   6.91e-01  1.06e+04        1    1.08e+00    2.21e+00
   2  3.670266e+10   -3.67e+10    1.84e+08   3.27e+03  -1.07e+04  5.30e+03        1    7.50e-01    2.96e+00
   3  4.335397e+07   -3.97e+07    1.84e+08   2.74e+03  -1.16e+01  1.32e+03        1    7.13e-01    3.67e+00
   4  1.345488e+06    2.34e+06    4.12e+07   1.55e+03   6.87e-01  1.40e+03        1    9.01e-01    4.57e+00
   5  5.376653e+05    8.08e+05    9.99e+06   6.64e+02   7.46e-01  1.59e+03        1    9.36e-01    5.51e+00

Change-Id: Iae538fe089be07c7bb219337a6f1392f7213acfe

internal/ceres/trust_region_minimizer.cc

@@ -83,6 +83,10 @@ void TrustRegionMinimizer::Minimize(const Minimizer::Options& options,
 
   while (FinalizeIterationAndCheckIfMinimizerCanContinue()) {
     iteration_start_time_in_secs_ = WallTimeInSeconds();
+
+    const double previous_gradient_norm = iteration_summary_.gradient_norm;
+    const double previous_gradient_max_norm = iteration_summary_.gradient_max_norm;
+
     iteration_summary_ = IterationSummary();
     iteration_summary_.iteration =
         solver_summary->iterations.back().iteration + 1;
@@ -111,12 +115,22 @@ void TrustRegionMinimizer::Minimize(const Minimizer::Options& options,
       return;
     }
 
+
+
     if (IsStepSuccessful()) {
       RETURN_IF_ERROR_AND_LOG(HandleSuccessfulStep());
-      continue;
-    }
+    } else {
+      // Declare the step unsuccessful and inform the trust region strategy.
+      iteration_summary_.step_is_successful = false;
+      iteration_summary_.cost = candidate_cost_ + solver_summary_->fixed_cost;
 
-    HandleUnsuccessfulStep();
+      // When the step is unsuccessful, we do not compute the gradient
+      // (or update x), so we preserve its value from the last
+      // successful iteration.
+      iteration_summary_.gradient_norm = previous_gradient_norm;
+      iteration_summary_.gradient_max_norm = previous_gradient_max_norm;
+      strategy_->StepRejected(iteration_summary_.relative_decrease);
+    }
   }
 }
 
@@ -793,12 +807,5 @@ bool TrustRegionMinimizer::HandleSuccessfulStep() {
   return true;
 }
 
-// Declare the step unsuccessful and inform the trust region strategy.
-void TrustRegionMinimizer::HandleUnsuccessfulStep() {
-  iteration_summary_.step_is_successful = false;
-  strategy_->StepRejected(iteration_summary_.relative_decrease);
-  iteration_summary_.cost = candidate_cost_ + solver_summary_->fixed_cost;
-}
-
 }  // namespace internal
 }  // namespace ceres

internal/ceres/trust_region_minimizer.h

@@ -81,7 +81,6 @@ class TrustRegionMinimizer : public Minimizer {
   bool MinTrustRegionRadiusReached();
 
   bool IsStepSuccessful();
-  void HandleUnsuccessfulStep();
   bool HandleSuccessfulStep();
   bool HandleInvalidStep();