[test] lr_find with bs_scale (#6422)

* init test: test_lr_find_with_bs_scale

* Update test_lr_finder.py

* remove gpu req

* try boring model

* custom boring model

* pep8

* fix typo

* Update test_lr_finder.py

* typo

* typo

Author: s-rog

tests/trainer/test_lr_finder.py

@@ -271,3 +271,26 @@ def test_lr_finder_fails_fast_on_bad_config(tmpdir):
     trainer = Trainer(default_root_dir=tmpdir, max_steps=2, auto_lr_find=True)
     with pytest.raises(MisconfigurationException, match='should have one of these fields'):
         trainer.tune(BoringModel())
+
+
+def test_lr_find_with_bs_scale(tmpdir):
+    """ Test that lr_find runs with batch_size_scaling """
+
+    class BoringModelTune(BoringModel):
+        def __init__(self, learning_rate=0.1, batch_size=2):
+            super().__init__()
+            self.save_hyperparameters()
+
+    model = BoringModelTune()
+    before_lr = model.hparams.learning_rate
+
+    # logger file to get meta
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=3,
+    )
+    bs = trainer.tuner.scale_batch_size(model)
+    lr = trainer.tuner.lr_find(model).suggestion()
+
+    assert lr != before_lr
+    assert isinstance(bs, int)