[!156][STREAMING] Add baseline FixedAudioHistorySelection for StreamST

# Why is the change needed?
To compare the new StreamAtt policy, it is useful to have a baseline that cuts the audio based on the number of textual history words discarded * fixed word duration (of 280ms, following previous work) to compare with.

# What changes does the patch introduce?
Adds a text-first history selection method that implements this logic:
- first, the new textual history is selected based on a fixed number of words to retain
- second, the number of words discarded (the difference between the textual history words of the previous step and the fixed number of words to retain) are multiplied by a fixed duration (here, 280ms) and these frames are cut from the audio history

# How was this patch tested?
UTs

examples/speech_to_text/simultaneous_translation/agents/v1_1/streaming/text_first_history_selection.py

@@ -16,7 +16,7 @@
 
 import torch
 
-from examples.speech_to_text.simultaneous_translation.agents.speech_utils import BOW_PREFIX
+from examples.speech_to_text.simultaneous_translation.agents.speech_utils import BOW_PREFIX, SHIFT_SIZE
 from examples.speech_to_text.simultaneous_translation.agents.v1_1.streaming.history_selection import HistorySelection
 from fairseq.data import Dictionary
 from fairseq.data.audio.speech_to_text_dataset import SpeechToTextDataset
@@ -194,3 +194,37 @@ def text_history(self, action: Action, states: AgentStates):
                     f"{self.history_max_len}")
             new_history = new_history[-self.history_max_len:]
         return new_history
+
+
+class FixedAudioHistorySelection(FixedWordsHistorySelection):
+    """
+    Audio history selection method that assign to each token of the textual history a fixed
+     duration of *FIXED_WORD_DURATION* and cut the audio history, stored in *states.source*,
+     accordingly. The history for the next decoding step is defined as follows:
+     - First, a pre-defined number of words (*history_words*) is retained as textual history from
+      the textual history of the previous decoding step and the *current_hypo* that is determined
+      by the SimulST agent and added to *states.target_indices*;
+     - Second, the new audio history is selected by discarding the audio frames corresponding to
+     the number of words discarded from the textual history multiplied by *FIXED_WORD_DURATION*.
+
+     The implementation works only for SentencePiece up to now.
+    """
+    FIXED_WORD_DURATION = 280   # duration of a word (in ms) as per (Ma et al., 2021)
+
+    def audio_history(self, action: Action, states: AgentStates, new_text_history: List[int]):
+        # Compute the number of words discarded from textual history
+        n_discarded_tokens = len(states.target_indices) - len(new_text_history)
+
+        # If no discarded tokens, return the original audio
+        if n_discarded_tokens == 0:
+            return states.source[0]
+
+        discarded_tokens = states.target_indices[:n_discarded_tokens]
+
+        n_discarded_words = len(self.tgt_dict.string(
+            discarded_tokens).strip(BOW_PREFIX).split(BOW_PREFIX))
+
+        # Recover the original number of frames considering that each audio feature corresponds
+        # to 10ms (SHIFT_SIZE)
+        frames_to_discard = n_discarded_words * self.FIXED_WORD_DURATION // SHIFT_SIZE
+        return states.source[0][frames_to_discard:]

fbk_simul_uts/v1_1/test_streamatt.py

@@ -19,7 +19,7 @@
 from examples.speech_to_text.simultaneous_translation.agents.v1_1.streaming.streaming_st_agent import StreamingSTAgent, \
     get_class_from_string
 from examples.speech_to_text.simultaneous_translation.agents.v1_1.streaming.text_first_history_selection import \
-    PunctuationHistorySelection
+    PunctuationHistorySelection, FixedAudioHistorySelection
 from simuleval.agents import ReadAction, WriteAction
 
 from fbk_simul_uts.v1_1.test_base_simulst_agent import BaseSTAgentTestCaseV2, MockedLoadModelVocab
@@ -218,6 +218,57 @@ def test_prefix_punctuation_selection(self, get_hypo_and_prefix):
         # Check first no frame discarded
         self.assertEqual(len(self.states.source[0]), 24)
 
+    @patch('examples.speech_to_text.simultaneous_translation.agents.v1_1.'
+           'simul_offline_alignatt.AlignAttSTAgent._get_hypo_and_prefix')
+    def test_fixed_audio_selection(self, get_hypo_and_prefix):
+        hypo = {
+            "tokens": torch.tensor([4, 5, 7, 8, 0]),  # I am quokka.
+            "attention": torch.tensor([
+                [0.5, 0.05, 0.05, 0.05, 0.05, 0.3],  # first frame mostly attended
+                [0.0, 0.6, 0.05, 0.03, 0.02, 0.3],  # second frame mostly attended
+                [0.05, 0.5, 0.05, 0.05, 0.05, 0.3],  # second frame mostly attended
+                [0.0, 0.6, 0.05, 0.03, 0.02, 0.3],  # second frame mostly attended
+                [0.05, 0.05, 0.05, 0.5, 0.05, 0.3],  # last frame mostly attended
+            ]).transpose(0, 1)
+        }
+
+        self.args.history_words = 1
+        self.agent.history_selection_method = FixedAudioHistorySelection(
+            self.agent.simulst_agent.tgtdict, self.agent.simulst_agent.args)
+
+        # No prefix
+        get_hypo_and_prefix.return_value = hypo, 0
+        self.states.target_indices = []
+        self.states.source = [torch.rand(280 // 10 * 4)]
+        action = self.agent.policy(self.states)
+        self.assertIsInstance(action, WriteAction)
+        self.assertEqual(action.content, "I am")
+        # "I am" should be written but only "am" should be retained as textual history (since
+        # history_words is set to 1), therefore 280ms (corresponding to one word) should be
+        # discarded
+        self.assertEqual(len(self.states.source[0]), 280 // 10 * 3)
+
+        # History len 1: "I"
+        get_hypo_and_prefix.return_value = hypo, 1
+        self.states.target_indices = [4]
+        self.states.source = [torch.rand(280 // 10 * 4)]
+        action = self.agent.policy(self.states)
+        self.assertIsInstance(action, WriteAction)
+        self.assertEqual(action.content, "am")
+        # "am" should be written and retained as textual history (since history_words is set to 1)
+        # while "I" should be discarded, therefore 280ms (corresponding to one word) should be
+        # discarded
+        self.assertEqual(len(self.states.source[0]), 280 // 10 * 3)
+
+        # History len 1: "am"
+        get_hypo_and_prefix.return_value = hypo, 2
+        self.agent.states.target_indices = [5]
+        self.states.source = [torch.rand(280 // 10 * 4)]
+        action = self.agent.policy(self.states)
+        self.assertIsInstance(action, ReadAction)
+        # Check no frame discarded
+        self.assertEqual(len(self.states.source[0]), 280 // 10 * 4)
+
     @patch('examples.speech_to_text.simultaneous_translation.agents.v1_1.'
            'simul_offline_alignatt.AlignAttSTAgent._get_hypo_and_prefix')
     def test_no_token_emitted(self, get_hypo_and_prefix):