Changed modeling_fx_utils.py to utils/fx.py for clarity
Moved **modeling_fx_utils.py** to **utils/fx.py** to make it clear that it is not "modeling_flax_utils.py".

Since there are a modeling_utils.py for PyTorch, and a modeling_tf_utils.py for TensorFlow, modeling_fx_utils.py could be believed to be the Flax counterpart, but it is actually related to the torch.fx feature, hence the pull request.