feat: do not spawn a thread for num_thread=1

[vikmary]

The spawning of a single thread on my computer produces a huge time overhead.

Thus I propose to add an if clause that would lead to no multithreading when num_thread=1 is used. This is already implemented for the case when multiple inputs are used, but isn't for the case when a single string is being input to .augment().

I have benchmarked with the following code:

import random
import time
import nlpaug.augmenter.char as nac

aug = nac.OcrAug()
times = []
for i in range(100):
    start = time.time()
    aug.augment(''.join(random.choice('qwertyuiopsdfghj klzxcv bnm,') for _ in range(random.randint(10, 30))))
    times.append(time.time() - start)
print(sum(times) / len(times))

For the dev version of nlpaug it results in the delta time of 0.10184303998947143, while the proposed version results in 9.496212005615235e-05, which makes huge difference.