Add persistent flag to Metric.add_state (#4195)

* add persistant flag to add_state in metrics

* wrap register_buffer with try catch

* pep8

* use loose version

* test

* pep8

pytorch_lightning/metrics/metric.py

@@ -17,6 +17,7 @@
 from collections.abc import Mapping, Sequence
 from collections import namedtuple
 from copy import deepcopy
+from distutils.version import LooseVersion
 
 import os
 import torch
@@ -78,7 +79,9 @@ def __init__(
         self._reductions = {}
         self._defaults = {}
 
-    def add_state(self, name: str, default, dist_reduce_fx: Optional[Union[str, Callable]] = None):
+    def add_state(
+        self, name: str, default, dist_reduce_fx: Optional[Union[str, Callable]] = None, persistent: bool = True
+    ):
         """
         Adds metric state variable. Only used by subclasses.
 
@@ -90,6 +93,7 @@ def add_state(self, name: str, default, dist_reduce_fx: Optional[Union[str, Call
                 If value is ``"sum"``, ``"mean"``, or ``"cat"``, we will use ``torch.sum``, ``torch.mean``,
                 and ``torch.cat`` respectively, each with argument ``dim=0``. The user can also pass a custom
                 function in this parameter.
+            persistent (Optional): whether the state will be saved as part of the modules ``state_dict``.
 
         Note:
             Setting ``dist_reduce_fx`` to None will return the metric state synchronized across different processes.
@@ -130,7 +134,11 @@ def add_state(self, name: str, default, dist_reduce_fx: Optional[Union[str, Call
             )
 
         if isinstance(default, torch.Tensor):
-            self.register_buffer(name, default)
+            if LooseVersion(torch.__version__) >= LooseVersion("1.6.0"):
+                # persistent keyword is only supported in torch >= 1.6.0
+                self.register_buffer(name, default, persistent=persistent)
+            else:
+                self.register_buffer(name, default)
         else:
             setattr(self, name, default)
 

tests/metrics/test_metric.py

@@ -1,5 +1,6 @@
 import pickle
 
+from distutils.version import LooseVersion
 import cloudpickle
 import numpy as np
 import pytest
@@ -59,6 +60,19 @@ def custom_fx(x):
     assert a._reductions["e"](torch.tensor([1, 1])) == -1
 
 
+def test_add_state_persistent():
+    a = Dummy()
+
+    a.add_state("a", torch.tensor(0), "sum", persistent=True)
+    assert "a" in a.state_dict()
+
+    a.add_state("b", torch.tensor(0), "sum", persistent=False)
+
+    if LooseVersion(torch.__version__) >= LooseVersion("1.6.0"):
+        assert "b" not in a.state_dict()
+
+
+
 def test_reset():
     class A(Dummy):
         pass