examples CI: shrink CPU DDP arm batch to fix gloo timeout

[wz337]

Summary:
The examples GitHub workflow's "Run DDP example on CPU" step launched two gloo
ranks each at local_batch_size=1024 on the 4-core 4-core-ubuntu-gpu-t4 runner.
The combined memory of two 1024-batch processes plus Shampoo preconditioner state
exceeds the runner, so one rank stalls/OOMs and the peer hangs until the 30-min
gloo recv timeout in backward:

RuntimeError: [gloo/transport/tcp/unbound_buffer.cc:78]
Timed out waiting 1800000ms for recv operation to complete
-> ChildFailedError: distributed_shampoo.examples.cifar10_example FAILED

This is a pre-existing latent flake: the example DDP code (cifar10_example.py,
examples/utils.py, examples/parallelism.py, ddp.yaml) is byte-identical to prior
syncs; it only surfaced once a run got past the CIFAR download. The single-process
CPU steps at batch 1024 pass -- it is the 2x memory of the DDP arm that tips over.

Fix: reduce the CPU DDP step to local_batch_size=128 (the config default), which
exercises the same DDP + gloo + Shampoo path at ~8x lower activation memory. The
GPU DDP step and the single-process steps are unchanged.

Differential Revision: D112602955

[meta-codesync]

@wz337 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112602955.