Introduce the ml_var_partition_pruning feature

Introduce the ml_var_partition_pruning feature

Add the ml_var_partition_pruning encoder speed feature that
uses a neural net model to prune partition-none and partition-split
search. The model uses prediction residue variance and quantization
step size as input features.

Encoding speed gain for speed 0(tested over 20 hdres clips):
            QP=30    QP=40
average     17.7%    18.3%
max        24.46%    26.6%

Coding loss:
lowres 0.071%;  midres 0.098%;  hdres 0.163%

Currently it is enabled for speed 0 low-bit depth only. It needs to be
tuned for other settings.

Change-Id: Ifb7417daa6bb6e7c97bb676269ce54ab0dc7b8c8