[PERFORMANCE] Improve vocab lookup performance by working with a dict…

…() directly (#1382) (#1385)

Co-authored-by: Sheng Zha <szha@users.noreply.github.com>

Co-authored-by: shishirb126 <71469961+shishirb126@users.noreply.github.com>
Co-authored-by: Sheng Zha <szha@users.noreply.github.com>

src/gluonnlp/vocab/vocab.py

@@ -30,7 +30,7 @@
 
 from .. import _constants as C
 from .. import embedding as emb
-from ..data.utils import Counter, DefaultLookupDict, count_tokens
+from ..data.utils import Counter, count_tokens
 
 UNK_IDX = 0
 _DEPR_PAD = object()
@@ -219,10 +219,7 @@ def __init__(self, counter: Optional[Counter] = None, max_size: Optional[int] =
         # Set up idx_to_token and token_to_idx based on presence of unknown token
         self._unknown_token = unknown_token
         self._idx_to_token = [unknown_token] if unknown_token else []
-        if unknown_token:
-            self._token_to_idx = DefaultLookupDict(UNK_IDX)
-        else:
-            self._token_to_idx = {}
+        self._token_to_idx = dict()
 
         # Handle special tokens
         special_tokens = []
@@ -267,10 +264,6 @@ def __init__(self, counter: Optional[Counter] = None, max_size: Optional[int] =
 
         if token_to_idx:
             self._sort_index_according_to_user_specification(token_to_idx)
-            if unknown_token:
-                self._token_to_idx._default = \
-                    self._token_to_idx[unknown_token]  # pytype: disable=not-writable
-
 
     def _index_counter_keys(self, counter, unknown_token, special_tokens, max_size,
                             min_freq):
@@ -395,9 +388,17 @@ def __getitem__(self, tokens):
         """
 
         if not isinstance(tokens, (list, tuple)):
-            return self._token_to_idx[tokens]
+            if self._unknown_token:
+                unknown_token_idx = self._token_to_idx[self._unknown_token]
+                return self._token_to_idx.get(tokens, unknown_token_idx)
+            else:
+                return self._token_to_idx[tokens]
         else:
-            return [self._token_to_idx[token] for token in tokens]
+            if self._unknown_token:
+                unknown_token_idx = self._token_to_idx[self._unknown_token]
+                return [self._token_to_idx.get(token, unknown_token_idx) for token in tokens]
+            else:
+                return [self._token_to_idx[token] for token in tokens]
 
     def __len__(self):
         return len(self._idx_to_token)