Add CHANGELOG entry for #21744

Author: c-pozzi

src/lightning/fabric/CHANGELOG.md

@@ -20,7 +20,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
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+- Fixed `_atomic_save` staging local checkpoints through `$TMPDIR`, which caused "no space left on device" on SLURM/HPC setups with a small `/tmp` ([#21744](https://github.com/Lightning-AI/pytorch-lightning/pull/21744))
 
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