Adding 2 new decoders: (#1196)

* Adding 2 new decoders:

- Fuse will simply concatenate all tokens into 1 string
- Strip will remove n char from left or right

Sequence(Replace("_", " "), Fuse(), Strip(1, 0)) should be what we want
for the `Metaspace` thing.

- Note: Added a new dependency from better parsing of decoders.
This is due to untagged enums which can match anything the `MustBe`
ensure there's no issue between Fuse and ByteFallback.
Since both are new the chances for backward incompatibility is low.

* Fixing picking/unpickling (using default args.).

* Stub.

* Black.

* Fixing node.

bindings/python/tests/bindings/test_decoders.py

@@ -13,6 +13,8 @@ from tokenizers.decoders import (
     WordPiece,
     ByteFallback,
     Replace,
+    Strip,
+    Fuse,
 )
 
 
@@ -94,6 +96,30 @@ class TestByteFallback:
         assert decoder.decode(["<0xE5>", "<0x8f>", "<0xab>", "a"]) == "叫a"
 
 
+class TestFuse:
+    def test_instantiate(self):
+        assert Fuse() is not None
+        assert isinstance(Fuse(), Decoder)
+        assert isinstance(Fuse(), Fuse)
+        assert isinstance(pickle.loads(pickle.dumps(Fuse())), Fuse)
+
+    def test_decoding(self):
+        decoder = Fuse()
+        assert decoder.decode(["My", " na", "me"]) == "My name"
+
+
+class TestStrip:
+    def test_instantiate(self):
+        assert Strip(left=0, right=0) is not None
+        assert isinstance(Strip(left=0, right=0), Decoder)
+        assert isinstance(Strip(left=0, right=0), Strip)
+        assert isinstance(pickle.loads(pickle.dumps(Strip(left=0, right=0))), Strip)
+
+    def test_decoding(self):
+        decoder = Strip(left=1, right=0)
+        assert decoder.decode(["My", " na", "me"]) == "ynae"
+
+
 class TestMetaspace:
     def test_instantiate(self):
         assert Metaspace() is not None