Fix weights by putting the right value in `lm_head.weight`

There was probably a bug in the initial conversion script that created those models, as the weights they have have a
different value for `lm_head.weight` and `model.decoder.embed_tokens.weight`. Those models are tied though.

This was not a problem until now as the model was tied after the load and the (wrong) value of `lm_head.weight` was
replaced by the value of `model.decoder.embed_tokens.weight`. This does not work any more if we tie the weights before
the load however, as the value picked might be the one from `lm_head.weight` depending on how the models are tied.
As far as I can see, the model stop generating properly on Transformers main.

This should fix the bug without any side effect.

pytorch_model.bin

@@ -1,3 +1,3 @@
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-size 211352677
+oid sha256:e51945d1b6e2691bd2f3a4501b4783c0b781ef2966bc99c43024c2f4789b9715
+size 149915781