| MergeIdLists: Merge multiple ID_LISTs
| into a single 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.
|
| Given multiple inputs of the form lengths_0,
| values_0, lengths_1, values_1, …
| which correspond to lengths and values
| of ID_LISTs of different features,
| this operator produces a merged ID_LIST
| that combines the ID_LIST features.
| The final merged output is described
| by a lengths and values vector.
|
| WARNING: The merge makes no guarantee
| about the relative order of ID_LISTs
| within a batch. This can be an issue if
| ID_LIST are order sensitive.
|