make sure top-level timer is closed before writing

[chriselion]

Proposed change(s)

This fixes a problem that was introduced in #3604. Since the TrainerController.start_learning isn't "closed" when we write the stats, we don't track any time in it. This results in timers that look like

    "total": 43845.636981294956,
    "count": 1,
    "self": 43845.23411354399,
    "children": {
        "run_training.setup": {
            "total": 0.40286775096319616,
            "count": 1,
            "self": 0.40286775096319616
        },
        "TrainerController.start_learning": {
            "total": 0.0,
            "count": 0,
            "self": 0.0,

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://jira.unity3d.com/browse/MLA-765 is a bigger-picture change for how to handle this; in the meantime, this moves the writing of timers into learn.py.

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments