Word Embedding init std adjustment

[hatanp]

This pull request adds two new arguments to adjust word embedding initialization:
--adjust-word-embedding-init which enables reading a std value from a new different variable,
and
--word-embedding-init-std which controls the init std if above flag is set.
I chose to have the enable flag so that we do not break any existing functionality in a case where a script is changing normal init std and thus the word embedding init std too. This way we retain that functionality as --adjust-word-embedding-init defaults to False if not set.

This does not adjust positional embeddings and they are currently initialized with the default initialization method.