[s2s] add create student script
This PR adds `create_student.py` script to create the student models for distillbart/pegasus/marian and t5.

```bash
python make_student.py \
  facebook/bart-large-cnn \
  --e 12 \
  --d 6 \
  --save_path student-bart-cnn-12-6 \
```
What do you think @patil-suraj ? 
+ Added t5 support, renamed create_bart_student -> make_student
+ made the code return which layers it copied so that distillation.py can use it.
+ Deleted a bunch of broken stuff!
+ added `save_randomly_initialized_model.py` script.
<|||||>LGTM, much cleaner now. 
Just left couple of nits, hope you don't mind me doing this :)<|||||>Love the nits!