update the end value of warmup

Summary:
Pull Request resolved: facebookresearch#4932

When `rescale_interval` is True, the end value of warmup should be the init lr (`schelduer(0.0)`) instead of `scheduler(warmup_length)`

Reviewed By: tglik

Differential Revision: D44323253

fbshipit-source-id: 8a5b1cdee4184dbdb999632ca090d0fd5073e2cb

detectron2/solver/lr_scheduler.py

@@ -42,7 +42,8 @@ def __init__(
             rescale_interval: whether we will rescale the interval of the scheduler after
                 warmup
         """
-        end_value = scheduler(warmup_length)  # the value to reach when warmup ends
+        # the value to reach when warmup ends
+        end_value = scheduler(0.0) if rescale_interval else scheduler(warmup_length)
         start_value = warmup_factor * scheduler(0.0)
         if warmup_method == "constant":
             warmup = ConstantParamScheduler(start_value)