Enable graph-level multiple streams execution.
See some related documentation here: https://docs.google.com/document/d/1yL3lWk_iFKqLTyekkuaiKXZ78I0lPmD5kM1fghHRs4Y/edit
This works as the basic implementation of the whole multi-stream feature, which is proposed in this [PR](https://github.com/tensorflow/tensorflow/pull/59843).

Multi-stream greatly speed up the execution of the model in high concurrency scenarios by putting different queries on different stream groups. The total number of stream groups to create can be set via the environment variable `TF_GPU_STREAM_GROUP_COUNT=N`, where `N` is a positive integer.