Downgrade datasets in axolotl example (#10849)

* Downgrade datasets to 2.15.0 to address axolotl prepare issue OpenAccess-AI-Collective/axolotl#1544

Tks to @kwaa for providing the solution in #10821 (comment)

python/llm/example/GPU/LLM-Finetuning/axolotl/README.md

@@ -22,6 +22,8 @@ git checkout v0.4.0
 cp ../requirements-xpu.txt requirements.txt
 pip install -e .
 pip install transformers==4.36.0
+# to avoid https://github.com/OpenAccess-AI-Collective/axolotl/issues/1544
+pip install datasets==2.15.0
 ```
 
 ### 2. Configures OneAPI environment variables and accelerate