Add Dream 7B Diffusion Large Language Model Pipeline

[dg845]

What does this PR do?

This PR implements a pipeline for the Dream 7B diffusion large language model (blog post, weights and code, repo). Dream is a masked (discrete) diffusion model for text which claims to perform comparably to similarly sized SOTA autoregressive LLMs such as Qwen 2.5 7B on NLP tasks and have superior performance on planning tasks.

Fixes #12017.

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