Fix type error when dividing chunk size in colossalai strategy (#16212)

Co-authored-by: awaelchli <aedu.waelchli@gmail.com>

src/pytorch_lightning/CHANGELOG.md

@@ -143,6 +143,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Enhanced `reduce_boolean_decision` to accommodate `any`-analogous semantics expected by the `EarlyStopping` callback ([#15253](https://github.com/Lightning-AI/lightning/pull/15253))
 
 
+- Fixed a type error when dividing the chunk size in the ColossalAI strategy ([#16212](https://github.com/Lightning-AI/lightning/pull/16212))
+
+
 ## [1.8.6] - 2022-12-21
 
 - minor cleaning

src/pytorch_lightning/strategies/colossalai.py

@@ -300,7 +300,8 @@ class ColossalAIStrategy(DDPStrategy):
                 min_chunk_size_mb: float = self.chunk_size_search_kwargs.get(
                     "min_chunk_size", 32 * 1024**2
                 )  # type: ignore[assignment]
-                min_chunk_size_mb /= 1024**2
+                if min_chunk_size_mb is not None:
+                    min_chunk_size_mb /= 1024**2
 
                 model = _LightningModuleWrapperBase(self.model)
                 self.model = GeminiDDP(