Add start index to metadata in TextSplitter (#5912)

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#### Add start index to metadata in TextSplitter

- Modified method `create_documents` to track start position of each
chunk
- The `start_index` is included in the metadata if the `add_start_index`
parameter in the class constructor is set to `True`

This enables referencing back to the original document, particularly
useful when a specific chunk is retrieved.

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#### Who can review?

Tag maintainers/contributors who might be interested:
@eyurtsev @agola11
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docs/modules/indexes/text_splitters/getting_started.ipynb

@@ -12,7 +12,8 @@
     "\n",
     "- `length_function`: how the length of chunks is calculated. Defaults to just counting number of characters, but it's pretty common to pass a token counter here.\n",
     "- `chunk_size`: the maximum size of your chunks (as measured by the length function).\n",
-    "- `chunk_overlap`: the maximum overlap between chunks. It can be nice to have some overlap to maintain some continuity between chunks (eg do a sliding window)."
+    "- `chunk_overlap`: the maximum overlap between chunks. It can be nice to have some overlap to maintain some continuity between chunks (eg do a sliding window).\n",
+    "- `add_start_index` : wether to include the starting position of each chunk within the original document in the metadata. "
    ]
   },
   {
@@ -49,6 +50,7 @@
     "    chunk_size = 100,\n",
     "    chunk_overlap  = 20,\n",
     "    length_function = len,\n",
+    "    add_start_index = True,\n",
     ")"
    ]
   },
@@ -62,8 +64,8 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "page_content='Madam Speaker, Madam Vice President, our First Lady and Second Gentleman. Members of Congress and' lookup_str='' metadata={} lookup_index=0\n",
-      "page_content='of Congress and the Cabinet. Justices of the Supreme Court. My fellow Americans.' lookup_str='' metadata={} lookup_index=0\n"
+      "page_content='Madam Speaker, Madam Vice President, our First Lady and Second Gentleman. Members of Congress and' metadata={'start_index': 0}\n",
+      "page_content='of Congress and the Cabinet. Justices of the Supreme Court. My fellow Americans.' metadata={'start_index': 82}\n"
      ]
     }
    ],
@@ -90,7 +92,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.1"
+   "version": "3.9.16"
   },
   "vscode": {
    "interpreter": {