examples : install torch-cpu for model conversion tool/example (#15475)

Signed-off-by: Jie Fu <jiefu@tencent.com>

examples/model-conversion/requirements.txt

@@ -1,3 +1,4 @@
+--extra-index-url https://download.pytorch.org/whl/cpu
 torch~=2.6.0
 torchvision~=0.21.0
 transformers~=4.55.0