Use past agrument for GPT2 to speed up decoding

[chiragjn]

As suggested in #61, I made changes to use the past argument for GPT2. I have not however made the equivalent changes for xlnet as mems behaves differently and under-documented for the moment.

I did some benchmarking on a 2 core CPU server with the following code

import time
import numpy as np
import nlpaug.augmenter.sentence as nas

def bench(text, return_past, model_path='gpt2', num_runs=50):
    aug = nas.ContextualWordEmbsForSentenceAug(model_path=model_path,  top_k=100, device='cpu')
    aug.model.return_past = return_past
    # Ignore the first time forward
    aug.insert(text)
    times = []
    for _ in range(num_runs):
        start = time.time()
        aug.insert(text)
        ptime = (time.time() - start)
        print(ptime)
        times.append(ptime)
    print('AVG:', np.mean(times))
    return np.mean(times)

bench(text='Hello darkness my old friend, I have come to', return_past=True, model_path='/opt/models/transformers/gpt2') 

bench(text='Hello darkness my old friend, I have come to', return_past=False, model_path='/opt/models/transformers/gpt2')

Since there is randomness involved in decoding, I also made a separate branch on top of these changes to use greedy decoding instead.
chiragjn@5a5a848

decoding	top_k random	top_k random	greedy	greedy
return_past	TRUE	FALSE	TRUE	FALSE
average	1.346138215	3.696006279	0.2176607323	0.3007589674

All numbers here: https://docs.google.com/spreadsheets/d/178VnHeBpHWz5lKHLbBuYTRPqxiWXa7-rpCje2wQH_i8/edit?usp=sharing

I leave the API design decisions up to you. Let me know how can I improve this pull request

[codecov-io]

Codecov Report

Merging #63 into master will decrease coverage by 36.35%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #63       +/-   ##
===========================================
- Coverage   46.58%   10.22%   -36.36%     
===========================================
  Files         124       48       -76     
  Lines        4317     1809     -2508     
===========================================
- Hits         2011      185     -1826     
+ Misses       2306     1624      -682

Impacted Files	Coverage Δ
test/augmenter/char/test_random_char.py	2.19% <0%> (-97.81%)	⬇️
test/flow/test_sometimes.py	5% <0%> (-95%)	⬇️
test/augmenter/word/test_random_word.py	5.55% <0%> (-94.45%)	⬇️
test/augmenter/char/test_char.py	6.45% <0%> (-93.55%)	⬇️
test/model/char/test_keyboard_model.py	6.52% <0%> (-93.48%)	⬇️
test/augmenter/char/test_ocr.py	6.66% <0%> (-93.34%)	⬇️
test/augmenter/char/test_keyboard.py	12.5% <0%> (-87.5%)	⬇️
test/augmenter/word/test_split.py	13.33% <0%> (-86.67%)	⬇️
nlpaug/base_augmenter.py	3.12% <0%> (-68.11%)	⬇️
test/augmenter/word/test_word.py	5.88% <0%> (-64.71%)	⬇️
... and 109 more

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