[Modular] guard ModularPipeline.blocks attribute

[yiyixuxu]

pipeline blocks are definitions - they're dynamically composable. you can add, remove, combine, or swap blocks.
Pipelines are stateful, and modifying the blocks attribute on a pipeline directly won't update the pipeline's internal state correctly.

This change makes blocks a property that returns a deepcopy, so users can't change the blocks and expect it to work. e.g. if you do something like this, the original pipeline would still work (currently it does not)

pipe = ModularPipeline.from_pretrained()
text_block = pipe.blocks.sub_blocks.pop("text_encoder")

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[sayakpaul]

Important fix! Let's also add a mini test case for this?