Sampler refactor

[alanakbik]

This PR adds a FlairSampler interface to better enable passing custom samplers to the ModelTrainer.

For instance, if you want to always shuffle your dataset in chunks of 5 to 10 sentences, you provide a sampler like this:

# your trainer
trainer: ModelTrainer = ModelTrainer(tagger, corpus)

# execute training run
trainer.train('path/to/experiment/folder',
              max_epochs=150,
              # sample data in chunks of 5 to 10
              sampler=ChunkSampler(block_size=5, plus_window=5)
              )

[yosipk]

👍

[alanakbik]

👍