| DenseVectorToIdList: Convert a blob
| with dense feature into a ID_LIST.
|
| An ID_LIST is a list of IDs (may be ints,
| often longs) that represents a single
| feature. As described in https://caffe2.ai/docs/sparse-operations.html,
| a batch of ID_LIST examples is represented
| as a pair of lengths and values where
| the lengths (int32) segment the values
| or ids (int32/int64) into examples.
|
| Input is a single blob where the first
| dimension is the batch size and the second
| dimension is the length of dense vectors.
| This operator produces a
|
| ID_LIST where out_values are the indices
| of non-zero entries and out_lengths
| are the number of non-zeros entries
| in each row.
|