Fix MLflowCallback end_run() and add support for tags and nested runs (#17130)
* ensure mlflow.end_run() is executed at end of training when mlflow.start_run() was executed by the callback
* add debug msg
* add support for MLFLOW_TAGS, MLFLOW_RUN_ID, and MLFLOW_NESTED_RUN
* update to support python 3.6+
* Validate env variables using ENV_VARS_TRUE_VALUES
* Empty-Commit