debug: when using Piecewise.learning_rate while total epochs < 30 (#2901)

ppcls/optimizer/learning_rate.py

@@ -406,11 +406,17 @@ class Piecewise(LRBase):
                  learning_rate=None,
                  **kwargs):
         if learning_rate:
-            decay_epochs = list(range(0, epochs, 30))[1:]
+            decay_epochs = list(range(0, epochs, 30))
             values = [
-                learning_rate * (0.1**i)
-                for i in range(len(decay_epochs) + 1)
+                learning_rate * (0.1**i) for i in range(len(decay_epochs))
             ]
+            # when total epochs < 30, decay_epochs and values should be
+            # [] and [lr] respectively, but paddle dont support.
+            if len(decay_epochs) == 1:
+                decay_epochs = [epochs]
+                values = [values[0], values[0]]
+            else:
+                decay_epochs = decay_epochs[1:]
             logger.warning(
                 "When 'learning_rate' of Piecewise has beed set, "
                 "the learning rate scheduler would be set by the rule that lr decay 10 times every 30 epochs. "