Source code for speechbrain.k2_integration.lattice_decoder

"""Different decoding graph algorithms for k2, be it HL or HLG (with G LM
and bigger rescoring LM).

This code was adjusted from icefall (

  * Pierre Champion 2023
  * Zeyu Zhao 2023
  * Georgios Karakasidis 2023

from pathlib import Path
from typing import Dict, List, Optional, Union
from collections import OrderedDict

from . import k2  # import k2 from ./
from speechbrain.utils.distributed import run_on_main
from import arpa_to_fst

import torch
import logging

from . import graph_compiler, utils

logger = logging.getLogger(__name__)

[docs] def get_decoding( hparams: Dict, graphCompiler: graph_compiler.GraphCompiler, device="cpu" ): """ This function reads a config and creates the decoder for k2 graph compiler decoding. There are the following cases: - HLG is compiled and LM rescoring is used. In that case, compose_HL_with_G and use_G_rescoring are both True and we will create for example G_3_gram.fst.txt and G_4_gram.fst.txt. Note that the 3gram and 4gram ARPA lms will need to exist under `hparams['lm_dir']`. - HLG is compiled but LM rescoring is not used. In that case, compose_HL_with_G is True and use_G_rescoring is False and we will create for example G_3_gram.fst.txt. Note that the 3gram ARPA lm will need to exist under `hparams['lm_dir']`. - HLG is not compiled (only use HL graph) and LM rescoring used. In that case, compose_HL_with_G is False and use_G_rescoring is True. Note that the 4gram ARPA lms will need to exist under `hparams['lm_dir']`. - HLG is not compiled (only use HL graph) and LM rescoring is not used. In that case, compose_HL_with_G is False and use_G_rescoring is False and we will not convert LM to FST. Arguments --------- hparams: dict The hyperparameters. graphCompiler: graph_compiler.GraphCompiler The graphCompiler (H) device : torch.device The device to use. Returns ------- Dict: decoding_graph: k2.Fsa A HL or HLG decoding graph. Used with a nnet output and the function `get_lattice` to obtain a decoding lattice `k2.Fsa`. decoding_method: Callable[[k2.Fsa], k2.Fsa] A function to call with a decoding lattice `k2.Fsa` (obtained after nnet output intersect with a HL or HLG). Retuns an FsaVec containing linear FSAs Example ------- >>> import torch >>> from speechbrain.k2_integration.losses import ctc_k2 >>> from speechbrain.k2_integration.utils import lattice_paths_to_text >>> from speechbrain.k2_integration.graph_compiler import CtcGraphCompiler >>> from speechbrain.k2_integration.lexicon import Lexicon >>> from speechbrain.k2_integration.prepare_lang import prepare_lang >>> from speechbrain.k2_integration.lattice_decoder import get_decoding >>> from speechbrain.k2_integration.lattice_decoder import get_lattice >>> batch_size = 1 >>> log_probs = torch.randn(batch_size, 40, 10) >>> log_probs.requires_grad = True >>> # Assume all utterances have the same length so no padding was needed. >>> input_lens = torch.ones(batch_size) >>> # Create a samll lexicon containing only two words and write it to a file. >>> lang_tmpdir = getfixture('tmpdir') >>> lexicon_sample = "hello h e l l o\\nworld w o r l d\\n<UNK> <unk>" >>> lexicon_file = lang_tmpdir.join("lexicon.txt") >>> lexicon_file.write(lexicon_sample) >>> # Create a lang directory with the lexicon and,, >>> prepare_lang(lang_tmpdir) >>> # Create a lexicon object >>> lexicon = Lexicon(lang_tmpdir) >>> # Create a random decoding graph >>> graph = CtcGraphCompiler( ... lexicon, ... log_probs.device, ... ) >>> decode = get_decoding( ... {"compose_HL_with_G": False, ... "decoding_method": "onebest", ... "lang_dir": lang_tmpdir}, ... graph) >>> lattice = get_lattice(log_probs, input_lens, decode["decoding_graph"]) >>> path = decode["decoding_method"](lattice)['1best'] >>> text = lattice_paths_to_text(path, lexicon.word_table) """ compose_HL_with_G = hparams.get("compose_HL_with_G") use_G_rescoring = ( hparams.get("decoding_method") == "whole-lattice-rescoring" ) caching = ( False if "caching" in hparams and hparams["caching"] is False else True ) if compose_HL_with_G or use_G_rescoring: lm_dir = Path(hparams["lm_dir"]) G_path = lm_dir / (hparams["G_arpa"].replace("arpa", "fst.txt")) G_rescoring_path = ( lm_dir / (hparams["G_rescoring_arpa"].replace("arpa", "fst.txt")) if use_G_rescoring else None ) if compose_HL_with_G: run_on_main( arpa_to_fst, kwargs={ "words_txt": Path(hparams["lang_dir"]) / "words.txt", "in_arpa": lm_dir / hparams["G_arpa"], "out_fst": G_path, "ngram_order": 3, # by default use 3-gram for HLG's LM "cache": caching, }, ) if use_G_rescoring: run_on_main( arpa_to_fst, kwargs={ "words_txt": Path(hparams["lang_dir"]) / "words.txt", "in_arpa": lm_dir / hparams["G_rescoring_arpa"], "out_fst": G_rescoring_path, "ngram_order": 4, # by default use 4-gram for rescoring LM "cache": caching, }, ) output_folder = None if "output_folder" in hparams: output_folder = output_folder if compose_HL_with_G: G = utils.load_G(G_path, cache=caching) decoding_graph = graphCompiler.compile_HLG( G, cache_dir=output_folder, cache=caching ) else: decoding_graph = graphCompiler.compile_HL( cache_dir=output_folder, cache=caching ) if hparams.get("decoding_method") == "whole-lattice-rescoring": G_rescoring = None if not isinstance(hparams["rescoring_lm_scale"], list): hparams["rescoring_lm_scale"] = [hparams["rescoring_lm_scale"]] def decoding_method(lattice: k2.Fsa) -> Dict[str, k2.Fsa]: """Get the best path from a lattice given rescoring_lm_scale.""" # Lazy load rescoring G (takes a lot of time) for developer happiness nonlocal G_rescoring if G_rescoring is None:"Decoding method: whole-lattice-rescoring")"Loading rescoring LM: {G_rescoring_path}") G_rescoring_pt = utils.load_G(G_rescoring_path, cache=caching) graphCompiler.lexicon.remove_G_rescoring_disambig_symbols( G_rescoring_pt ) G_rescoring = utils.prepare_rescoring_G(G_rescoring_pt) # rescore_with_whole_lattice returns a list of paths depending on # lm_scale values. return rescore_with_whole_lattice( lattice, G_rescoring, lm_scale_list=hparams["rescoring_lm_scale"], ) elif hparams.get("decoding_method") in ["1best", "onebest"]:"Decoding method: one-best-decoding") def decoding_method(lattice: k2.Fsa) -> Dict[str, k2.Fsa]: """Get the best path from a lattice.""" return OrderedDict({"1best": one_best_decoding(lattice)}) else: def decoding_method(lattice: k2.Fsa): """A dummy decoding method that raises an error.""" raise NotImplementedError( f"{hparams.get('decoding_method')} not implemented as a decoding_method" ) return { "decoding_graph":, "decoding_method": decoding_method, }
[docs] @torch.no_grad() def get_lattice( log_probs_nnet_output: torch.Tensor, input_lens: torch.Tensor, decoder: k2.Fsa, search_beam: int = 5, output_beam: int = 5, min_active_states: int = 300, max_active_states: int = 1000, ac_scale: float = 1.0, subsampling_factor: int = 1, ) -> k2.Fsa: """ Get the decoding lattice from a decoding graph and neural network output. Arguments --------- log_probs_nnet_output: It is the output of a neural model of shape `(batch, seq_len, num_tokens)`. input_lens: It is an int tensor of shape (batch,). It contains lengths of each sequence in `log_probs_nnet_output`. decoder: It is an instance of :class:`k2.Fsa` that represents the decoding graph. search_beam: Decoding beam, e.g. 20. Ger is faster, larger is more exact (less pruning). This is the default value; it may be modified by `min_active_states` and `max_active_states`. output_beam: Beam to prune output, similar to lattice-beam in Kaldi. Relative to best path of output. min_active_states: Minimum number of FSA states that are allowed to be active on any given frame for any given intersection/composition task. This is advisory, in that it will try not to have fewer than this number active. Set it to zero if there is no constraint. max_active_states: Maximum number of FSA states that are allowed to be active on any given frame for any given intersection/composition task. This is advisory, in that it will try not to exceed that but may not always succeed. You can use a very large number if no constraint is needed. ac_scale: acoustic scale applied to `log_probs_nnet_output` subsampling_factor: The subsampling factor of the model. Returns ------- lattice: An FsaVec containing the decoding result. It has axes [utt][state][arc]. """ device = log_probs_nnet_output.device input_lens = if decoder.device != device: logger.warn( "Decoding graph (HL or HLG) not loaded on the same device" " as nnet, this will cause decoding speed degradation" ) decoder = input_lens = (input_lens * log_probs_nnet_output.shape[1]).round().int() # NOTE: low ac_scales may results in very big lattices and OOM errors. log_probs_nnet_output *= ac_scale lattice = k2.get_lattice( log_probs_nnet_output, input_lens, decoder, search_beam=search_beam, output_beam=output_beam, min_active_states=min_active_states, max_active_states=max_active_states, subsampling_factor=subsampling_factor, ) return lattice
[docs] @torch.no_grad() def one_best_decoding( lattice: k2.Fsa, use_double_scores: bool = True, ) -> k2.Fsa: """ Get the best path from a lattice. Arguments --------- lattice: The decoding lattice returned by :func:`get_lattice`. use_double_scores: True to use double precision floating point in the computation. False to use single precision. Returns ------- best_path: An FsaVec containing linear paths. """ best_path = k2.shortest_path(lattice, use_double_scores=use_double_scores) return best_path
[docs] @torch.no_grad() def rescore_with_whole_lattice( lattice: k2.Fsa, G_with_epsilon_loops: k2.Fsa, lm_scale_list: Optional[List[float]] = None, use_double_scores: bool = True, ) -> Union[k2.Fsa, Dict[str, k2.Fsa]]: """ Intersect the lattice with an n-gram LM and use shortest path to decode. The input lattice is obtained by intersecting `HLG` with a DenseFsaVec, where the `G` in `HLG` is in general a 3-gram LM. The input `G_with_epsilon_loops` is usually a 4-gram LM. You can consider this function as a second pass decoding. In the first pass decoding, we use a small G, while we use a larger G in the second pass decoding. Arguments --------- lattice: k2.Fsa An FsaVec with axes [utt][state][arc]. Its `aux_labels` are word IDs. It must have an attribute `lm_scores`. G_with_epsilon_loops: k2.Fsa An FsaVec containing only a single FSA. It contains epsilon self-loops. It is an acceptor and its labels are word IDs. lm_scale_list: Optional[List[float]] If none, return the intersection of `lattice` and `G_with_epsilon_loops`. If not None, it contains a list of values to scale LM scores. For each scale, there is a corresponding decoding result contained in the resulting dict. use_double_scores: bool True to use double precision in the computation. False to use single precision. Returns ------- If `lm_scale_list` is None, return a new lattice which is the intersection result of `lattice` and `G_with_epsilon_loops`. Otherwise, return a dict whose key is an entry in `lm_scale_list` and the value is the decoding result (i.e., an FsaVec containing linear FSAs). """ assert G_with_epsilon_loops.shape == (1, None, None) G_with_epsilon_loops = device = lattice.device if hasattr(lattice, "lm_scores"): lattice.scores = lattice.scores - lattice.lm_scores # We will use lm_scores from G, so remove lats.lm_scores here del lattice.lm_scores assert hasattr(G_with_epsilon_loops, "lm_scores") # Now, lattice.scores contains only am_scores # inv_lattice has word IDs as labels. # Its `aux_labels` is token IDs inv_lattice = k2.invert(lattice) num_seqs = lattice.shape[0] b_to_a_map = torch.zeros(num_seqs, device=device, dtype=torch.int32) # NOTE: The choice of the threshold list is arbitrary here to avoid OOM. # You may need to fine tune it. prune_th_list = [1e-10, 1e-9, 1e-8, 1e-7, 1e-6] prune_th_list += [1e-5, 1e-4, 1e-3, 1e-2, 1e-1] max_loop_count = 10 loop_count = 0 while loop_count <= max_loop_count: try: if device == "cpu": rescoring_lattice = k2.intersect( G_with_epsilon_loops, inv_lattice, treat_epsilons_specially=True, ) else: rescoring_lattice = k2.intersect_device( G_with_epsilon_loops, inv_lattice, b_to_a_map, sorted_match_a=True, ) rescoring_lattice = k2.top_sort(k2.connect(rescoring_lattice)) break except RuntimeError as e:"Caught exception:\n{e}\n") if loop_count >= max_loop_count: "Return None as the resulting lattice is too large." ) return None f"num_arcs before pruning: {inv_lattice.arcs.num_elements()}" ) "This OOM is not an error. You can ignore it. " "If your model does not converge well, or the segment length " "is too large, or the input sound file is difficult to " "decode, you will meet this exception." ) inv_lattice = k2.prune_on_arc_post( inv_lattice, prune_th_list[loop_count], True, ) f"num_arcs after pruning: {inv_lattice.arcs.num_elements()}" ) loop_count += 1 # lat has token IDs as labels # and word IDs as aux_labels. lat = k2.invert(rescoring_lattice) if lm_scale_list is None: return lat ans = OrderedDict() saved_am_scores = lat.scores - lat.lm_scores for lm_scale in lm_scale_list: am_scores = saved_am_scores / lm_scale lat.scores = am_scores + lat.lm_scores best_path = k2.shortest_path(lat, use_double_scores=use_double_scores) key = f"whole_lattice_rescore_lm_scale_{lm_scale:.1f}" ans[key] = best_path return ans