feat: pass use_fast parameter to get_tokenizer

[nikitajz]

A proposed solution for the issue #105

[felixgwu]

Looks great! There are only a few tiny changes that need to be made.

[nikitajz]

@felixgwu after a few attempts I'm still getting failed tests due to OOM (OSError: [Errno 12] Cannot allocate memory) even for old tests without a fast tokenizer. I've seen the related issue but lost the link that Travis CI decreased memory limit to 3.5 Gb (from 7Gb) which might be related. Though tests pass successfully on my laptop (except for one unrelated test due to commented model 'scibert-scivocab-uncased' which I mentioned earlier).
Apart from that, seems get_idf_dict worth refactoring (at least for fast tokenizers) to use tokenizer.encode_batch instead of paralleling tokenizer.encode, see related issue

[felixgwu]

Everything looks good to me now. I can either merge it or wait if you would like to spend more time on fixing the Travis CI. I really appreciate your help!

[nikitajz]

ok, let see if I can devote more time to fixing the tests, otherwise, feel free to merge it.
Also, I'd recommend changing the model in the tests from roberta-large to something lighter, e.g. distilroberta-base

[nikitajz]

I've refactored the tests a bit. I also added the missing assert statement to test_multi_refs_working, please check if it is correct. I temporarily added skip to test test_score_en_sci which fails due to commented model in utils. All tests pass successfully locally.
Unfortunately, I didn't fix failing tests on Travis CI quickly. As far as I can tell, there are two possible options:

• migrate to another CI hosting with more powerful VMs in the free tier
• replace model roberta-large in the test with lightweight, something like distilroberta-base, but this requires also changing some tests behaviour, e.g. test_multi_refs where a model is not specified explicitly, but chosen by language specified. Hence I didn't proceed with this option.

[felixgwu]

It looked good to me, so I just merged them. I would like to keep using roberta-large in the tests for now since it is the default model. Thanks again for your help!