Use synthetic CIFAR data in AlexNet tests (#8143)
The nightly full test consistently fails because of CIFAR dataset
download
([log](https://github.com/deepspeedai/DeepSpeed/actions/runs/29319047783)).
This PR uses deterministic CIFAR-shaped synthetic images and labels by
default so the AlexNet pipeline tests do not depend on an external
dataset download. Preserve explicit paths for offline or downloaded real
CIFAR-10 data.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
