[core] gracefully error out when attn-backend x cp combo isn't supported.

[sayakpaul]

What does this PR do?

We should be able to error out when an attention backend isn't supported with CP.

Refer to https://github.com/huggingface/diffusers/pull/12829#issuecomment-3645237672 and https://github.com/huggingface/diffusers/pull/12829#issuecomment-3645582823.

Additionally, we specify parallel_config to from_pretrained() and then set an attention backend through set_attention_backend() (which CP-incompatible), it goes through silently.

This PR eliminates those and gracefully errors out.

[HuggingFaceDocBuilderDev]

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