[cli] report all quant backends in diffusers-cli env.

[sayakpaul]

What does this PR do?

Report all quantization backends in diffusers-cli env to make it more comprehensive. It now unfolds to something like:

Copy-and-paste the text below in your GitHub issue and FILL OUT the two last points.

- 🤗 Diffusers version: 0.39.0.dev0
- Platform: Linux-5.15.0-1048-aws-x86_64-with-glibc2.31
- Running on Google Colab?: No
- Python version: 3.12.12
- PyTorch version (GPU?): 2.11.0+cu129 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Huggingface_hub version: 1.11.0
- Transformers version: 5.6.0.dev0
- Accelerate version: 1.12.0
- PEFT version: 0.18.2.dev0
- bitsandbytes version: 0.49.0
- gguf version: 0.19.0
- optimum-quanto version: not installed
- torchao version: 0.17.0
- nvidia-modelopt version: not installed
- Safetensors version: 0.7.0
- xFormers version: not installed
- Accelerator: NVIDIA H100 80GB HBM3, 81559 MiB
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

[DN6]

If the backend isn't available, I'd say it's better to skip adding it to the final message entirely. Keeps the logs less noisy.

[sayakpaul]

Done.

[HuggingFaceDocBuilderDev]

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