global: add possibility to set flair seed

[stefan-it]

Hi,

this PR adds the possibility to set a seed via wrapping the Hugging Face Transformers library helper method.

By specifying a seed with:

import flair

flair.set_seed(42)

more reproducable experiments are possible. The wrapped set_seed method sets seeds for random, numpy and torch. More details here.

[alanakbik]

@stefan-it thanks for adding this!