Lexiconfree beam search

[SimBe195]

Simple time synchronous beam search algorithm based on the new SearchAlgorithmV2 interface. Does not use (proper) pronunciation lexicon, word-level LM or transition model. Performs special handling of blank if a blank index is set. Main purpose is open vocabulary search with CTC/Neural Transducer (or similar) models.

Supports global pruning by max beam-size and by score difference to the best hypothesis. Uses a LabelScorer to context initialization/extension and scoring.

The search requires a lexicon that represents the vocabulary. Each lemma is viewed as a token with its index in the lexicon corresponding to the associated output index of the LabelScorer.

[larissakl]

Two general points:

1. How should this search algorithm be used? Are you planning to put the enum SearchTypeV2, the related ParameterChoice and Module_::createSearchAlgorithm() to Module.hh/.cc later? Or do you have a different plan?
2. Would it maybe be a good idea to factor out the struct TimeStatistic and its related code so that it can be reused in other search algorithms?

[SimBe195]

1. How should this search algorithm be used? Are you planning to put the `enum SearchTypeV2`, the related `ParameterChoice` and `Module_::createSearchAlgorithm()` to Module.hh/.cc later? Or do you have a different plan?

With this PR alone, the search algorithm is not usable yet. I will make PR's for an Flf node and python bindings separately. But I can include the createSearchAlgorithm() function already here.

2. Would it maybe be a good idea to factor out the `struct TimeStatistic` and its related code so that it can be reused in other search algorithms?

Yeah, probably. Maybe even into Core?