Add ml_var_partition experiment

Add ml_var_partition experiment

Make partition decisions using machine learning models. The goal is to
achieve better coding quality than the variance-based parititioning
without much encoding speed loss.

To enable this experiment, use --enable-ml-var-partition for config.

When eanbled, the variance-based partitioning is replaced by this ML
based partitioing for speed 6 and above in real time mode(except low
resolution or high bit-depth).

Current coding gains(average PSNR):
                speed 6      speed 7      speed 8
rtc              2.04%        2.65%        3.90%
ytlivehr         3.11%        4.53%       11.57%
hdres(rtc mode)  5.10%

Further testing and tuning is needed to see if the speed and quality
tradeoff is reasonable.

Change-Id: I0da5a2fbc22c3261832b32920ee36d9b19d417af