9. A computer program product for determining compliance of content with a content policy, the computer program product comprising a non-transitory computer-readable storage medium containing computer program code for: receiving a content item comprising text and one or more images; extracting a plurality of text signals from the text; extracting a plurality of image signals from the one or more images; inputting the plurality of text signals and the plurality of image signals into a two-tier classifier system by inputting the plurality of text signals into a text classifier model of a first tier of the two-tier classifier system, inputting the plurality of image signals into an image classifier model of the first tier of the two tier-classifier system, and inputting output classifications of the text classifier model and of the image classifier model into a second-tier classifier model, the second-tier classifier outputting a confidence value expressing likelihood of compliance with a content policy of an online system; comparing the confidence value against a pre-defined threshold value; and based on the comparison, assigning a compliance classification to the content item.
9. A computer program product for determining compliance of content with a content policy, the computer program product comprising a non-transitory computer-readable storage medium containing computer program code for: receiving a content item comprising text and one or more images; extracting a plurality of text signals from the text; extracting a plurality of image signals from the one or more images; inputting the plurality of text signals and the plurality of image signals into a two-tier classifier system by inputting the plurality of text signals into a text classifier model of a first tier of the two-tier classifier system, inputting the plurality of image signals into an image classifier model of the first tier of the two tier-classifier system, and inputting output classifications of the text classifier model and of the image classifier model into a second-tier classifier model, the second-tier classifier outputting a confidence value expressing likelihood of compliance with a content policy of an online system; comparing the confidence value against a pre-defined threshold value; and based on the comparison, assigning a compliance classification to the content item. 11. The computer program product of claim 9 , further comprising: detecting commonly appearing word pairs; and treating each word pair as a single text signal.