Fix get_last_lr value

get_lr() was deprecated and instead the method intended to be used for getting the correct lr is get_last_lr()

Author: 1adrianb

warmup_scheduler/scheduler.py

@@ -57,6 +57,7 @@ def step(self, epoch=None, metrics=None):
                     self.after_scheduler.step(None)
                 else:
                     self.after_scheduler.step(epoch - self.total_epoch)
+                self._last_lr = self.after_scheduler.get_last_lr()
             else:
                 return super(GradualWarmupScheduler, self).step(epoch)
         else: