`BackTranslationAugmenter` produces an incorrect number of transformations

[aemartinez]

Describe the bug
Calling the augment function of a BackTranslationAugmenter produces a list with a length that appears to be non-deterministic, sometimes not matching the value of the transformations_per_example initialization parameter.

To Reproduce
Run the following code:

import textattack
augmenter = textattack.augmentation.recipes.BackTranslationAugmenter(transformations_per_example=5)
s = "You are in the kitchen."
ls = augmenter.augment(s)
print(len(ls))

Expected behavior
Expected output: 5
Actual output: 3

System Information (please complete the following information):

• OS: MacOS Sonoma 14.6.1
• Library versions: torch==2.3.1, transformers==4.42.4
• Textattack version: textattack==0.3.10
• Full pip freeze output: https://pastebin.com/VPt7JdRY