support to specify priority about steps and epochs in LinearWarmup (#8564)

for uapi

ppdet/optimizer/optimizer.py

@@ -165,17 +165,20 @@ class LinearWarmup(object):
             of `epochs` is higher than `steps`. Default: None.
     """
 
-    def __init__(self, steps=500, start_factor=1. / 3, epochs=None):
+    def __init__(self, steps=500, start_factor=1. / 3, epochs=None, epochs_first=True):
         super(LinearWarmup, self).__init__()
         self.steps = steps
         self.start_factor = start_factor
         self.epochs = epochs
+        self.epochs_first = epochs_first
 
     def __call__(self, base_lr, step_per_epoch):
         boundary = []
         value = []
-        warmup_steps = self.epochs * step_per_epoch \
-            if self.epochs is not None else self.steps
+        if self.epochs_first and self.epochs is not None:
+            warmup_steps = self.epochs * step_per_epoch
+        else:
+            warmup_steps = self.steps
         warmup_steps = max(warmup_steps, 1)
         for i in range(warmup_steps + 1):
             if warmup_steps > 0: