Cleanup Megatron

- [ ] [Organize examples](https://docs.google.com/document/d/1FC6a8B5aYfOSwBkj3w45q8ReLuQuXI06Z3dvyJpNvUE/edit?tab=t.0#heading=h.f9i8b7des7zb)
- This includes removing examples that are outdated (e.g. bert)
- [x] [Remove deprecated features](https://docs.google.com/spreadsheets/d/1UiWfzcB8QxMIaL_VtbOdvqlfj4hvY7XK5iZI7uXar8I/edit?gid=0#gid=0)
- i.e. features that have been marked as deprecated for a long period of time or marked for removal by a past release
- https://github.com/NVIDIA/Megatron-LM/pull/3410
- [x] Mark unused files and features for deprecation
- [x] Create a list for each expert group and get approval
- [x] Execute approved list
- [ ] Remove pickle due to security concerns ([more context](https://gitlab-master.nvidia.com/ADLR/megatron-lm/-/issues/536))