Documentation: Add SMP API changes

Author: anirudh2290

doc/api/training/smd_model_parallel.rst

@@ -43,6 +43,7 @@ Select a version to see the API documentation for version. To use the library, r
 .. toctree::
    :maxdepth: 1
 
+   smp_versions/v1_2_0.rst
    smp_versions/v1_1_0.rst
 
 It is recommended to use this documentation alongside `SageMaker Distributed Model Parallel