FX submodule naming fix
This PR is related to HFTracer, the class responsible for allowing torch.fx symbolic tracing on transformers models.

It enhances the way dynamically inserted modules are named, making the name of the submodule inserted to the parent more explicit and close to what the submodule represents. It also solves issues related to the way submodule were inserted: instead of using `setattr`, `nn.Module.add_module()` is used.