Added immutable keyword argument to canonical_form function

[kaneki003]

Description

-Fixes #39177. Avoiding extra code block for declaring an immutable graph while generating it.
-Added immutable keyword argument to canonical_form function to directly return an immutable canonical label of a given graph.
-Updated the function to handle the immutable argument and return an immutable graph if specified.
-Added tests to verify the functionality of the immutable argument.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[dcoudert]

this is not enough. You must also add parameter immutable to method canonical_label in generic_graph.py and ensure that it is also working well when algorithm='sage'.