Fix norm in retokenizer split (#6111)

Parallel to behavior in merge, reset norm on original token in
retokenizer split.

spacy/tests/doc/test_retokenize_split.py

@@ -198,3 +198,22 @@ def test_doc_retokenizer_realloc(en_vocab):
         token = doc[0]
         heads = [(token, 0)] * len(token)
         retokenizer.split(doc[token.i], list(token.text), heads=heads)
+
+
+def test_doc_retokenizer_split_norm(en_vocab):
+    """#6060: reset norm in split"""
+    text = "The quick brownfoxjumpsoverthe lazy dog w/ white spots"
+    doc = Doc(en_vocab, words=text.split())
+
+    # Set custom norm on the w/ token.
+    doc[5].norm_ = "with"
+
+    # Retokenize to split out the words in the token at doc[2].
+    token = doc[2]
+    with doc.retokenize() as retokenizer:
+      retokenizer.split(token, ["brown", "fox", "jumps", "over", "the"], heads=[(token, idx) for idx in range(5)])
+
+    assert doc[9].text  == "w/"
+    assert doc[9].norm_ == "with"
+    assert doc[5].text  == "over"
+    assert doc[5].norm_ == "over"

spacy/tokens/_retokenize.pyx

@@ -355,6 +355,7 @@ def _split(Doc doc, int token_index, orths, heads, attrs):
         lex = doc.vocab.get(doc.mem, orth)
         token.lex = lex
         token.lemma = 0  # reset lemma
+        token.norm = 0  # reset norm
         if to_process_tensor:
             # setting the tensors of the split tokens to array of zeros
             doc.tensor[token_index + i] = xp.zeros((1,doc.tensor.shape[1]), dtype="float32")