set overwrite default value to True in add_symbol

This ensures compatibility with all the calls to add_symbol across the repo (which overwrite by default, as in the original implementation). The only place where the value is explicitly changed is when loading the dictionary from file (which was the source of the bug). In a file you have to explicitly say whether the tokens should be overwritten or duplicated

fairseq/data/dictionary.py

@@ -124,7 +124,7 @@ def unk_string(self, escape=False):
         else:
             return self.unk_word
 
-    def add_symbol(self, word, n=1, overwrite=False):
+    def add_symbol(self, word, n=1, overwrite=True):
         """Adds a word to the dictionary"""
         if word in self.indices and overwrite:
             idx = self.indices[word]
@@ -337,7 +337,7 @@ def encode_line(
 
         for i, word in enumerate(words):
             if add_if_not_exist:
-                idx = self.add_symbol(word, overwrite=True)
+                idx = self.add_symbol(word)
             else:
                 idx = self.index(word)
             if consumer is not None:
@@ -367,7 +367,7 @@ def _add_file_to_dictionary_single_worker(
     def add_file_to_dictionary(filename, dict, tokenize, num_workers):
         def merge_result(counter):
             for w, c in sorted(counter.items()):
-                dict.add_symbol(w, c, overwrite=True)
+                dict.add_symbol(w, c)
 
         local_file = PathManager.get_local_path(filename)
         offsets = find_offsets(local_file, num_workers)