Fix token text splitter duplicates (#14848)

- **Description:** 
- Add a break case to `text_splitter.py::split_text_on_tokens()` to
avoid unwanted item at the end of result.
    - Add a testcase to enforce the behavior.
  - **Issue:** 
    - #14649 
    - #5897
  - **Dependencies:** n/a,
 
---

**Quick illustration of change:**

```
text = "foo bar baz 123"

tokenizer = Tokenizer(
        chunk_overlap=3,
        tokens_per_chunk=7
)

output = split_text_on_tokens(text=text, tokenizer=tokenizer)
```
output before change: `["foo bar", "bar baz", "baz 123", "123"]`
output after change: `["foo bar", "bar baz", "baz 123"]`

libs/langchain/langchain/text_splitter.py

@@ -670,6 +670,8 @@ def split_text_on_tokens(*, text: str, tokenizer: Tokenizer) -> List[str]:
     chunk_ids = input_ids[start_idx:cur_idx]
     while start_idx < len(input_ids):
         splits.append(tokenizer.decode(chunk_ids))
+        if cur_idx == len(input_ids):
+            break
         start_idx += tokenizer.tokens_per_chunk - tokenizer.chunk_overlap
         cur_idx = min(start_idx + tokenizer.tokens_per_chunk, len(input_ids))
         chunk_ids = input_ids[start_idx:cur_idx]

libs/langchain/tests/unit_tests/test_text_splitter.py

@@ -13,6 +13,8 @@
     MarkdownHeaderTextSplitter,
     PythonCodeTextSplitter,
     RecursiveCharacterTextSplitter,
+    Tokenizer,
+    split_text_on_tokens,
 )
 
 FAKE_PYTHON_TEXT = """
@@ -1175,3 +1177,18 @@ def test_html_header_text_splitter(tmp_path: Path) -> None:
     docs_from_file = splitter.split_text_from_file(tmp_path / "doc.html")
 
     assert docs_from_file == expected
+
+
+def test_split_text_on_tokens() -> None:
+    """Test splitting by tokens per chunk."""
+    text = "foo bar baz 123"
+
+    tokenizer = Tokenizer(
+        chunk_overlap=3,
+        tokens_per_chunk=7,
+        decode=(lambda it: "".join(chr(i) for i in it)),
+        encode=(lambda it: [ord(c) for c in it]),
+    )
+    output = split_text_on_tokens(text=text, tokenizer=tokenizer)
+    expected_output = ["foo bar", "bar baz", "baz 123"]
+    assert output == expected_output