Fix XLNet and Transformer-XL Execution

[tiagokv]

If you execute the following snippet exception OverflowError: int too big to convert is thrown:

from flair.embeddings import TransformerWordEmbeddings
from flair.data import Sentence

model = TransformerWordEmbeddings("xlnet-large-cased", "all", "mean", layer_mean=False)
model.embed(Sentence("This is a test that will not work"))

This happens because both XLNet and Transformer-XL do not have a max sequence length and tokenizer.model_max_length comes with a very big integer.

[alanakbik]

@tiagokv thanks for fixing this!