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:e7e09074b2434d761da8191f9a798ed1195986c83eb79f7f8d348df73b839b49
-size 586638339
+oid sha256:7bc7e1973431752bcbb083c090c3c8f1e6eb3389d2e2e0faf9bf1a85d969a789
+size 469780741