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 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:27bef0a4fce2ae1f4bf5408c6c1453e1bc984d1ecdc62e0455488b8fac465987
-size 221610115
+oid sha256:e7202aa461d7acc323b2fe553f34f31a4038d187b19ffc9046ec897cb2eeb32f
+size 155052933