Tensorboard documentation

[mansishr]

Might fix #519
I see a scope of improvement for this documentation. Suggestions will be helpful.

[psfoley]

@mansishr Thanks for starting on this. A couple other things that should be part of this PR:

• Tensorboard logs can also be captured with the TaskRunner API, and the documentation should indicate that. A description of write_logs parameters in the plan (see here). When it's set to true, tensorboard logs will be written.
• Explain what metrics get saved by the aggregator (aggregator and collaborator metrics), and how to launch tensorboard from the workspace