Avoid re-computing model parameter count every iteration

[jaketae]

In training.py, we have

https://github.com/bigscience-workshop/Megatron-DeepSpeed/blob/fd1e1da967c74e598acfc011031474663ef5845e/megatron/training.py#L818

However, this appears to be wasted compute since the model parameter count does not change. We can refactor the code so that get_parameters_in_billions is called only once.