Changelog: add explanation r. QLoRA mem savings

CHANGELOG.md

 ### 0.43.2
 
+This release is quite significant as the QLoRA bug fix big implications for higher `seqlen` and batch sizes.
+
+For each sequence (i.e. batch size increase of one) we expect memory savings of:
+- 405B: 39GB for `seqlen=1024`, and 4888GB for `seqlen=128,00`
+- 70B: 10.1GB for `seqlen=1024` and  1258GB for `seqlen=128,00`
+
+This was due to activations being unnecessary for frozen parameters, yet the memory for them was still erroneously allocated due to the now fixed bug.
+
 #### Improvements:
 
 - docs: FSDP+QLoRA and CPU install guide (#1211 #1227, thanks @stevhliu)