With the v5 rc being out, we should now ensure that the PEFT tests pass.
This PR contains fixes to achieve that.

1. hub_online_once was failing because
transformers.utils.hub._is_offline_mode no longer exists. Using the new
function instead if transformers v5 is detected.

2.
tests/test_encoder_decoder_models.py::TestEncoderDecoderModels::test_merge_layers[LoraConfig-config_kwargs10-peft-internal-testing/tiny-random-BartForConditionalGeneration]
failing due to TrainableTokensWrapper not being applied to all layers
owing to changes to _tied_weights_keys.

3. While working on this, I discovered a tangential bug in
TrainableTokensLayer.get_merged_weights. This method returns a
torch.Tensor but the expected type is nn.Parameter (since foo.bar.weight
is supposed to be a nn.Parameter). This type mismatch would cause
torch's model.get_parameter, which I used in
_get_module_names_tied_with_embedding, to fail. At first, I wanted to
change the return type to nn.Parameter but this causes all kinds of
issues. Therefore, I left this bug as is. Instead, in
_get_module_names_tied_with_embedding, I opted to use attrgetter instead
of model.get_parameter.