Add AdapterSetup context manager

[calpt]

Background

All adapter-supporting model classes currently provide an adapter_names parameter in their forward() call that allows dynamic & state-less configuration of the activated adapters. We tried to remove this parameter with the release of v2.0.0, however it turned out to be essential for a multi-threaded inference use-case (see #165 for context). The removal was therefore reverted shortly after (#176; v.2.0.1).

Problem

The adapter_names parameter has to be passed down all module classes to the actual adapter layers. This means that the method headers of all top-level and intermediate module classes need to be modified which makes maintanance difficult & prevents further disentangling AH & HF.

Suggested solution

As a replacement to the adapter_names parameter, this PR introduces a new AdapterSetup class. This class is intended to be used as a context manager, i.e. a typical use case would look like this:

# will use no adapters
outputs = model(**inputs)

with AdapterSetup(Stack("a", "b")):
    # will use the adapter stack "a" and "b"
    outputs = model(**inputs)

Note that in the above example no adapters are activated via active_adapters. Within the with block, the adapter implementation will read the currently active setup from the context manager.

This solution allows dynamic adapter activation, e.g. also with nesting:

with AdapterSetup(Stack("a", "b")):
    # will use the adapter stack "a" and "b"
    outputs = model(**inputs)
    with AdapterSetup(Fuse("c", "d"), head_setup="e"):
        # will use fusion between "c" and "d" & head "e"
        outputs = model(**inputs)

Most importantly, the context manager is thread-local, i.e. we can use different setups in different threads with the same model instance. There's a test for this:
https://github.com/calpt/adapter-transformers/blob/93065b74ae933b34ec5c3f4047223792a795239f/tests/test_adapter_setup_context.py#L75-L112

Breaking changes

• The adapter_names parameter is removed for all model classes