Update dependencies of machine translation modules

modules/text/machine_translation/transformer/en-de/requirements.txt

-nltk==3.4.5
+sacremoses
 subword-nmt

modules/text/machine_translation/transformer/en-de/utils.py

@@ -16,14 +16,7 @@ import re
 from typing import List
 
 import codecs
-try:
-    import nltk
-    nltk.data.find('misc/perluniprops')
-    nltk.data.find('corpora/nonbreaking_prefixes')
-except LookupError:
-    nltk.download('perluniprops')
-    nltk.download('nonbreaking_prefixes')
-from nltk.tokenize.moses import MosesTokenizer, MosesDetokenizer
+from sacremoses import MosesTokenizer, MosesDetokenizer
 from subword_nmt.apply_bpe import BPE
 
 

modules/text/machine_translation/transformer/zh-en/requirements.txt

-nltk==3.4.5
 jieba
+sacremoses
 subword-nmt

modules/text/machine_translation/transformer/zh-en/utils.py

@@ -21,14 +21,7 @@ import jieba
 
 jieba.setLogLevel(logging.INFO)
 
-try:
-    import nltk
-    nltk.data.find('misc/perluniprops')
-    nltk.data.find('corpora/nonbreaking_prefixes')
-except LookupError:
-    nltk.download('perluniprops')
-    nltk.download('nonbreaking_prefixes')
-from nltk.tokenize.moses import MosesDetokenizer
+from sacremoses import MosesTokenizer, MosesDetokenizer
 from subword_nmt.apply_bpe import BPE