Make reprojection layer optional

[alanakbik]

This PR makes the reprojection layer optional in SequenceTagger. You can control this behavior through the reproject_embeddings parameter. If you set it to True, embeddings are reprojected via linear map to identical size. If set to False, no reprojection happens. If you set this parameter to an integer, the linear map maps embedding vectors to vectors of this size.

# tagger with standard reprojection
tagger = SequenceTagger(
    hidden_size=256,
    [...]
    reproject_embeddings=True,
)

# tagger without reprojection
tagger = SequenceTagger(
    hidden_size=256,
    [...]
    reproject_embeddings=False,
)

# reprojection to vectors of length 128
tagger = SequenceTagger(
    hidden_size=256,
    [...]
    reproject_embeddings=128,
)