[discrete diffusion] Add dflash pipeline

[kashif]

What does this PR do?

Added the DFlash pipeline as a stanalone PR extracted from #12911

Fixes # (issue)

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[dg845]

Hi, when I try out the example in src/diffusers/pipelines/dflash/pipeline_dflash.py:

import torch
from diffusers import DFlashPipeline
from transformers import AutoModel, AutoModelForCausalLM, AutoTokenizer

draft = AutoModel.from_pretrained(
    "z-lab/Qwen3-8B-DFlash-b16", trust_remote_code=True, dtype=torch.bfloat16
)
target = AutoModelForCausalLM.from_pretrained("Qwen/Qwen3-8B", dtype=torch.bfloat16)
tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen3-8B")

pipe = DFlashPipeline(draft_model=draft, target_model=target, tokenizer=tokenizer)
out = pipe(prompt="How many positive whole-number divisors does 196 have?")
print(out.texts[0])

I get the following output:

<think>
Okay, so I need to figure out how many positive whole-number divisors 196 has. Hmm, divisors... right. Divisors are numbers that divide into 196 without leaving a remainder. So, I guess I need to find all the numbers that can divide 196 evenly.

First, maybe I should start by recalling how to find the number of divisors. I think there's a method involving prime factorization. Let me remember... if you break down a number into its prime factors, then you can use the exponents of those primes to calculate the total number of divisors.

Let me try that. So, first step: find the prime factors of 196. Let me start dividing 196 by the smallest prime numbers.

196 is even, so it's divisible by 2. Let me divide 196 by 2.

196 ÷ 2 = 98.

Okay, so 2 is a prime factor. Now, 98 is also even, so divide by 2 again.

98 ÷ 2 = 49.

So now we have 2 × 2 × 49. Now, 49 is not even, so let's check the next prime number, which is 3. Does 3 divide into 49? Let me check: 3 × 16 is 48, so 49 - 48 is 1. So no, 3 doesn't divide into 49.

Next prime number is 5. 49 ends with a 9, so it's not divisible by 5. Next is 7. Let me try dividing

Is the output being cut off expected?

[dg845]

Also, I saw that huggingface/transformers#45846 was closed, will the KV cache work correctly without it? DFlashTokenDiffusionScheduler has some cache methods (e.g. snapshot_cache, restore_cache, etc.) but I'm not sure if they need the transformers PR to work.

[kashif]

thanks @dg845 checking