Allow Peft models to share their base model

[fozziethebeat]

Why are these changes needed?

This adds a special environment variable that activates shared Peft model base weights. Currently when loading two Peft models that have the same base model, those model weights are loaded once. With this flag activated, all Peft models will share the same base model.

To make this work it requires a few work around due to how Huggingface's Peft model has implemented LoRA adapters, the most popular variant. These modify the base model's pytorch modules directly and thus adapters sharing the same base model must live within the same model object and a set_adapter method must be called to switch between them.

Related issue number (if applicable)

Expands #1805

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).

[BabyChouSr]

This is pretty cool! I tested this using the following script, and it seems to work well!

I served multiple LoRAs using the following script:

PEFT_SHARE_BASE_WEIGHTS=true python3 -m fastchat.serve.multi_model_worker \
    --model-path /data/chris/peft-llama-dummy-1 \
    --model-names peft-dummy-1 \
    --model-path /data/chris/peft-llama-dummy-2 \
    --model-names peft-dummy-2 \
    --model-path /data/chris/peft-llama-dummy-3 \
    --model-names peft-dummy-3 \
    --num-gpus 2

Looking at the GPU utilization, we only load the base model once, so in my case, we only load llama-7b once so ~14GB VRAM, which is what we expect.

[fozziethebeat]

thank goodness it works for someone else. I've submitted a few too many things that didn't 100% work. I have it running as well on GCP with two modes and they give distinct results and fit within VRAM as expected.

[Ying1123]

LGTM. Thanks!