Exponential Parameter Scheduler (#715)

Summary:
Pull Request resolved: facebookresearch/ClassyVision#715

Exponential parameter scheduler for fvcore/ClassyVision. This type of scheduler is useful as a LR scheduler when using EMA.

Differential Revision: D27058657

fbshipit-source-id: 176ee5bcd074d5d517afa660a7dbf86f83468979

fvcore/common/param_scheduler.py

@@ -7,6 +7,7 @@
     "ParamScheduler",
     "ConstantParamScheduler",
     "CosineParamScheduler",
+    "ExponentialParamScheduler",
     "LinearParamScheduler",
     "CompositeParamScheduler",
     "MultiStepParamScheduler",
@@ -85,6 +86,33 @@ def __call__(self, where: float) -> float:
         )
 
 
+class ExponentialParamScheduler(ParamScheduler):
+    """
+    Exponetial schedule based on start value and decay, where value is caluated
+    for timestep t out of T total stepsm by
+    param_t = start_value * (decay ** t/T).The schedule is updated after every train
+    step by default based on the fraction of samples seen.
+
+    Example:
+
+        .. code-block:: python
+            ExponentialParamScheduler(start_value=2.0, decay=0.02)
+
+    Corresponds to a decreasing schedule with values in [2.0, 0.04).
+    """
+
+    def __init__(
+        self,
+        start_value: float,
+        decay: float,
+    ) -> None:
+        self._start_value = start_value
+        self._decay = decay
+
+    def __call__(self, where: float) -> float:
+        return self._start_value * (self._decay ** where)
+
+
 class LinearParamScheduler(ParamScheduler):
     """
     Linearly interpolates parameter between ``start_value`` and ``end_value``.

tests/param_scheduler/test_scheduler_exponential.py

@@ -0,0 +1,29 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+
+import unittest
+
+from fvcore.common.param_scheduler import ExponentialParamScheduler
+
+
+class TestExponentialScheduler(unittest.TestCase):
+    _num_epochs = 10
+
+    def _get_valid_config(self):
+        return {"start_value": 2.0, "decay": 0.1}
+
+    def _get_valid_intermediate_values(self):
+        return [1.5887, 1.2619, 1.0024, 0.7962, 0.6325, 0.5024, 0.3991, 0.3170, 0.2518]
+
+    def test_scheduler(self):
+        config = self._get_valid_config()
+
+        scheduler = ExponentialParamScheduler(**config)
+        schedule = [
+            round(scheduler(epoch_num / self._num_epochs), 4)
+            for epoch_num in range(self._num_epochs)
+        ]
+        expected_schedule = [
+            config["start_value"]
+        ] + self._get_valid_intermediate_values()
+
+        self.assertEqual(schedule, expected_schedule)