* Add freeze as an argument

I train on different platforms and sometimes I want to freeze some layers. I have to go into the code and change it and also keep track of how many layers I froze on what platform. Please add the number of layers to freeze as an argument in future versions thanks.

* Update train.py

* Update train.py

* Cleanup
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>