[Tutorials] Handle various types of cell source

This is a duplicate of the same code change in botorch: pytorch/botorch#2752

I was noticing that sometimes (depending on a variety of factors like the editor used to modify the tutorial, whether they had been executed) the conversion script would fail because the cell source would be stored as a list of strings instead of a multi-line string.

From the official nbformat documentation:

> Some fields, such as code input and text output, are characteristically multi-line strings. When these fields are written to disk, they may be written as a list of strings, which should be joined with '' when reading back into memory. In programmatic APIs for working with notebooks (Python, Javascript), these are always re-joined into the original multi-line string. If you intend to work with notebook files directly, you must allow multi-line string fields to be either a string or list of strings.

https://ipython.readthedocs.io/en/3.x/notebook/nbformat.html

scripts/convert_ipynb_to_mdx.py

@@ -338,6 +338,25 @@ def sanitize_mdx(mdx: str) -> str:
     return mdx
 
 
+def get_source(cell: NotebookNode) -> str:
+    """
+    Extract the source code from a Jupyter notebook cell. The source is
+    characteristically multi-line strings but may be stored as a list of strings, in
+    which case we join them together.
+    https://ipython.readthedocs.io/en/3.x/notebook/nbformat.html
+
+    Args:
+        cell (NotebookNode): A Jupyter notebook cell object.
+
+    Returns:
+        str: The source code as a string.
+    """
+    source = cell.get("source", "")
+    if isinstance(source, list):
+        return "".join(source)
+    return source
+
+
 def handle_markdown_cell(
     cell: NotebookNode,
     new_img_dir: Path,
@@ -355,7 +374,7 @@ def handle_markdown_cell(
     Returns:
         str: Transformed Markdown object suitable for inclusion in MDX.
     """
-    markdown = cell["source"]
+    markdown = get_source(cell)
 
     # Update image paths in the Markdown and copy them to the Markdown tutorials folder.
     # Skip - Our images are base64 encoded, so we don't need to copy them to the docs
@@ -389,7 +408,7 @@ def handle_cell_input(cell: NotebookNode, language: str) -> str:
     Returns:
         str: Code block formatted Markdown string.
     """
-    cell_source = cell.get("source", "")
+    cell_source = get_source(cell)
     return f"```{language}\n{cell_source}\n```\n\n"