Created by: panyx0718

Timeline has been a go-place for TF developers when doing performance profiling. It visualizes the multi-device executions as different time-series. Arrows can be generated for cross device data transfer. Additional features such as memory allocation/deallocations are also very useful.

Some examples:

This is the first PR for the timeline feature.

1. It collects cuda kernel execution stats with user-defined names.
2. It stores the stats into a proto for future analysis.

Near-term next steps:

1. Some more clean up and collect other cuda events such as memcpy.
2. Generate timeline visualization with the protobuf.