fix LinearLrWarmup bug; test=develop (#24913)

python/paddle/fluid/dygraph/learning_rate_scheduler.py

@@ -664,6 +664,7 @@ class LinearLrWarmup(LearningRateDecay):
                 format(learning_rate))
         self.learning_rate = learning_rate
         self.warmup_steps = warmup_steps
+        self.start_lr = start_lr
         assert end_lr > start_lr, "end_lr {} must be greater than start_lr {}".format(
             end_lr, start_lr)
         self.lr_ratio_before_warmup = (
@@ -676,7 +677,7 @@ class LinearLrWarmup(LearningRateDecay):
 
         from .. import layers
         if self.step_num < self.warmup_steps:
-            return self.lr_ratio_before_warmup * self.step_num
+            return self.lr_ratio_before_warmup * self.step_num + self.start_lr
         else:
             return base_lr