[device_map] fix device_map check behaviour.

[sayakpaul]

What does this PR do?

Follow-up of #9449. It introduced a bug when doing:

diffusers/src/diffusers/pipelines/pipeline_utils.py

Line 412 in 41e4779

key for key, component in self.components.items() if model_has_device_map(component)

For models that have sharded checkpoints (such as FluxTransformer2DModel), we will always set device_map:

diffusers/src/diffusers/models/modeling_utils.py

Line 883 in 41e4779

device_map = {"": "cpu"}

So, this PR:

1. When a device map only has CPU we need to account for it. We also add a test for it in this PR.
2. Reuse model_has_device_map() functionality.

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Unrelated test failures.

[sayakpaul]

Closing in light of #9827.