Summary:
**Description**
Provide DeepText model with the functionality to load a secondary index (pre-trained char-ngram embedding, e.g. FastText) during training/test. Embeddings of out-of-vocabulary words will be computed on-the-fly during training/test by averaging the char-ngram embeddings.
**Approach**
This diff provides two custom operators to accomplish this task – ConditionalOp and IndexCharNgramGetOp. We first use IndexCharNgramGetOp to perform char-ngram index lookup and return a sparse tensor segmented by lengths for each token. The sparse tensor is then used to compute the average embedding provided by the char-ngram index. Finally, we use a ConditionalOp to replace those whose embeddings were not found in the original index during the feature apply stage. Please refer to documentations of the code for more details.
Reviewed By: jamesr66a
Differential Revision: D5666924
fbshipit-source-id: f76605d093154a014d5b9ebf9510de9d79874eee
