More robust handling of variadics in external

[BenjaminMidtvedt]

Fixes an issue where classes with both *args and **kwargs in their signature could not be created using Layer.

There is only very rare case that is not supported, which is when you have both variadics and positionaal only arguments. Example:

class MyOverlyComplexClass
    def __init__(self, posonly, /, param, *args):
        ...

layer = Layer(MyOverlyComplexClass, "foo", "bar", "baz")

Where we now throw a type error. Should be extremely rare (/ is almost never used). But in these cases you can provide a wrapper like

def overly_complex_builder(self, posonly, param, *args):
    return MyOverlyComplexClass(posonly, param, *args)

layer = Layer(overly_complex_builder, "foo", "bar", "baz")

[BenjaminMidtvedt]

Note that either variadics or positional only on their own are ok:

class MyLessComplexClass:
    def __init__(self, arg, *args, kwonly, param=None, **kwargs):
        ...

class OtherLessComplexClass:
    def __init__(self, a, /, b *, kw, **kwargs):
        ...

[BenjaminMidtvedt]

This should also close #24

[Henrik-KM]

Looks good, everything seems to work now at least with standard layers currently in torch.