fix changelog format (#21460)

update

Author: deependujha

src/lightning/pytorch/CHANGELOG.md

@@ -6,22 +6,29 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ---
 
-### Fixed
+## [Unreleased] - YYYY-MM-DD
+
+
+### Added
 
-- Fixed ``ModelParallelStrategy`` single-file checkpointing when ``torch.compile`` wraps the model so optimizer states no longer raise ``KeyError`` during save ([#21357](https://github.com/Lightning-AI/pytorch-lightning/issues/21357))
 -
 
+
 ### Deprecated
 
 - Deprecated `to_torchscript` method due to deprecation of TorchScript in PyTorch ([#21397](https://github.com/Lightning-AI/pytorch-lightning/pull/21397))
 
+
 ### Removed
 
----
 - Removed support for Python 3.9 due to end-of-life status ([#21398](https://github.com/Lightning-AI/pytorch-lightning/pull/21398))
 
+
 ### Fixed
 
+- Fixed ``ModelParallelStrategy`` single-file checkpointing when ``torch.compile`` wraps the model so optimizer states no longer raise ``KeyError`` during save ([#21357](https://github.com/Lightning-AI/pytorch-lightning/issues/21357))
+
+
 - Sanitize profiler filenames when saving to avoid crashes due to invalid characters ([#21395](https://github.com/Lightning-AI/pytorch-lightning/pull/21395))
 
 
@@ -31,6 +38,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fix `_generate_seed_sequence_sampling` function not producing unique seeds ([#21399](https://github.com/Lightning-AI/pytorch-lightning/pull/21399))
 
 
+---
+
+
 ## [2.6.0] - 2025-11-28
 
 ### Added