Support simpler syntax for specifying pipeline splits

[payoto]

What does this PR do?

This PR is a WIP

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

[payoto]

class IPUUNet2DConditionModel(UNet2DConditionModel, PipelineMixin):
    pipeline_splits = [
        ("conv_in", 0),
        ("down_blocks[2].downsamplers[0]", 1),
        ("up_blocks[0].resnets[2]", 2),
        ("up_blocks[1].attentions[2]", 3),
    ]
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def parallelize(self):
        super().parallelize()
        print(self.pipeline_splits)
        for layer, ipu_id in self.pipeline_splits:
            self.add_block(layer, ipu_id)
        return self

[HuggingFaceDocBuilderDev]

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