Fix tf shared embedding
# What does this PR do?
A hack was used to properly import the shared embedding weights, but it can be removed (removing it also is convenient for the sharding PR) 

Found this while testing #17713. In HF's `save_pretrained` and  `load_pretrained` the layer name is changed using ` name = "/".join(weight_name.split("/")[1:])`. This was breaking for OPT as the layer name was ` 'decoder.embed_tokens/model.decoder.embed_tokens/weight:0'` instead of `'tfopt_model/model/decoder/embed_tokens/weight:0'`. The naming is strange and had to use a scope hack. The hack comes from `BART`
All the TF weights of OPT will need to be updated if this is approved. I think I can handle that along with #17713. 
<|||||>_The documentation is not available anymore as the PR was closed or merged._