fix batch auto scaling when init_val causes OOM (#8954)

* fix batch auto scaling when `init_val` causes OOM

* Update CHANGELOG.md

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -197,6 +197,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed bug where data-loading functions where not getting the correct running stage passed ([#8858](https://github.com/PyTorchLightning/pytorch-lightning/pull/8858))
 
+- Fixed a bug in the binary search mode of auto batch size scaling where exception was thrown if the first trainer run resulted in OOM ([#8954](https://github.com/PyTorchLightning/pytorch-lightning/pull/8954))
+
 
 ## [1.4.0] - 2021-07-27
 

pytorch_lightning/tuner/batch_size_scaling.py

@@ -174,6 +174,7 @@ def _run_binsearch_scaling(
     """Batch scaling mode where the size is initially is doubled at each iteration
     until an OOM error is encountered. Hereafter, the batch size is further
     refined using a binary search"""
+    low = 1
     high = None
     count = 0
     while True: