Add cfg.SOLVER.NUM_DECAYS in the default config

Summary:
Pull Request resolved: #4535

cfg.SOLVER.NUM_DECAYS is needed for the WarmupStepWithFixedGammaLR schedule.

Reviewed By: wat3rBro

Differential Revision: D39363550

fbshipit-source-id: b2844d4c84b8538f59a958984627b9463b6d6324

Author: Xiaofang Wang

detectron2/config/defaults.py

@@ -541,6 +541,8 @@
 _C.SOLVER.GAMMA = 0.1
 # The iteration number to decrease learning rate by GAMMA.
 _C.SOLVER.STEPS = (30000,)
+# Number of decays in WarmupStepWithFixedGammaLR schedule
+_C.SOLVER.NUM_DECAYS = 3
 
 _C.SOLVER.WARMUP_FACTOR = 1.0 / 1000
 _C.SOLVER.WARMUP_ITERS = 1000