Set up a basic MLflow setup

Replicate all the tensorboard logging in Meg-DS, plus logging hyperparams of choice.
So on the code level:

1. repeat tensorboard 1:1 but log using mlflow api
2. find new places where to log new things (e.g. hyperparams)
3. WGs that want to log specific events/data will add those directly to Meg-DS code base
4. Currently the config is just `--mlflow-dir` on/off toggle which will log **all** MLFlow events/data

example:
https://gist.github.com/tsaoyu/14e39a6d246cb29b107a2cc62a12f7a3

**Blocking events:**

- [ ] @JetRunner setting up the MLFlow server