In benchark_cnn.py turn on the ScopedAllocator optimization whenever the variable update mode is collective_all_reduce.  Also, leave the number of device-to-device copy streams at 1, since increasing does not seem to help.
In benchark_cnn.py turn on the ScopedAllocator optimization whenever
the variable update mode is collective_all_reduce.  Also, leave
the number of device-to-device copy streams at 1, since increasing
does not seem to help.

PiperOrigin-RevId: 215406332

tensorflow/benchmarks,annarev/benchmarks,annarev/benchmarks,tensorflow/benchmarks