FIX: Adding 2 adapters when target_modules is a str fails

[BenjaminBossan]

Problem description

Adding two adapters (e.g. LoRA) when using a list for target_mdules works but passing a str fails. The issue is that for str, we do a re.fullmatch, whereas for list, we just check endswith. After adding the first adapter, though, the naming pattern of the modules changes. In the example above, the name for the linear layer changes from "lin0" to "base_model.model.lin0", which is why the fullmatch fails but the endswith still works.

Reproduction

from peft import LoraConfig, get_peft_model
from torch import nn

class MLP(nn.Module):
    def __init__(self, bias=True):
        super().__init__()
        self.lin0 = nn.Linear(10, 20, bias=bias)

def test_target_modules_list():
    config = LoraConfig(target_modules=["lin0"])
    test_it(config)
    print("Adding two adapters with target_module being a list works")

def test_target_modules_str():
    config = LoraConfig(target_modules="lin0")
    test_it(config)

def test_it(config):
    model = MLP()
    model = get_peft_model(model, config, "adapter0")
    model.add_adapter("adapter1", config)
    print("Adding two adapters with target_module being a str works")

if __name__ == "__main__":
    # works
    test_target_modules_list()
    # ValueError: Target modules lin0 not found in the base model
    test_target_modules_str()

I think that most users would be surprised that:

1. Adding the first adapter works but adding the second fails, even though they use the same config.
2. Using target_modules=["lin0"] works but target_modules="lin0" fails for the 2nd adapter.

Solution

We could change the logic of not using re.fullmatch for str, but I think that could be tricky to achieve without breaking BC. Instead, I chose to change the inject_adapter call in add_adapter to pass the base model, not the whole peft model. This way, the naming pattern is preserved.

Tests

Tests were added by removing the guard from #1105. This guard was added because of this bug and was how it was discovered first.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[pacman100]

Thank you @BenjaminBossan for fixing this bug! Nice find.