…e#1111)

* Fix adding 2 adapters when target_modules is a str

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

I haven't added extra tests for this. The script above could serve as a
test. However, it will be sufficient to remove the guard added in huggingface#1105:

    if isinstance(config.target_str, modules):
        # TODO this should be doable
        self.skipTest("Multiple adapters cannot currently be added when target_modules is a string.")

as that will test exactly this behavior and was how the bug was
originally uncovered. Depending on what PR lands first, the guard has to
removed in this PR or in huggingface#1105.

* Enable tests for adding 2 adapters with str