Log message passing in AutoML child contexts

### Method of passing log messages
To support the the ability to cancel runs, we needed a separate context for the child runs, and we pass the log messages from the child to parent.

Current method of passing the messages is imperfect.

**Current returned log format:**
`"[Source=AutoML, Kind=Trace] [Source=ImageClassificationTrainer; ImageClassificationTrainer, Kind=Trace] Phase: Bottleneck Computation, Dataset used: Train, Image Index: 3954"`

This duplicates the header on the messages (`[Source=AutoML, Kind=Trace]` and `[Source=ImageClassificationTrainer; ImageClassificationTrainer, Kind=Trace]`), and makes it difficult to filter/select to the wanted log lines.

**Correct format:**
`"[Source=AutoML; ImageClassificationTrainer; ImageClassificationTrainer, Kind=Trace] Phase: Bottleneck Computation, Dataset used: Train, Image Index: 3954"`

This single header then lets you filter on the log line properties (e.g. `e.Source == "xyz"`) instead of parsing its free text.