Rectangle box trainning InterpolateLabelOp

[yxlao]

Changes:

• box_size_x and box_size_y separate: this allows one samle prediction to conver a rectangular region, speeding up prediction.
• InterpolateLabel TF op for much faster label interpolation

Current run time on KITTI:

{'load_data': 0.009927988052368164, 
'predict': 0.04781484603881836,
'interpolate': 0.0362849235534668,
'write_data': 0.2045590877532959}