ML based rectangular partition search pruning

ML based rectangular partition search pruning

Add a ML model to predict if rectangular partition search can be skipped
without much coding loss. This model is enabled for speed 0 low bitdepth
only.

Impact on coding performance is minor:
             avg_psnr     ovr_psnr     ssim
lowres       -0.005%       0.005%     0.017%
midres        0.100%       0.114%     0.134%
hdres         0.048%       0.083%     0.074%
jvet480p      0.035%       0.027%     0.044%
jvet720p      0.094%       0.090%     0.174%

Tested encoding speed over 20 midres and hdres clips, average speed
gain is about 8%; maximum speed gain is 23%.

Change-Id: I5d4029dec7134c53ac68ab6cf0c8077dc0b767ed