tests : add script to benchmark whisper.cpp on LibriSpeech corpus (#2999)

* tests : add script to benchmark whisper.cpp on LibriSpeech corpus

LibriSpeech is a widely-used benchmark dataset for training and
testing speech recognition models.

This adds a set of scripts to measure the recognition accuracy of
whisper.cpp models, following the common benchmark standards.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

* Document how to prepare `whisper-cli` and model files

Feedback from Daniel Bevenius.

This adds a short code example how to prepare the `whisper-cli`
command, to make the initial setup step a little bit clearer.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

* tests : Simplify how to set up Python environment

Based on a feedback from Georgi Gerganov.

Instead of setting up a virtual environment in Makefile, let users
set up the Python environment. This is better since users may have
their own preferred workflow/toolkit.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

---------

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

tests/librispeech/eval.py

@@ -0,0 +1,47 @@
+import os
+import glob
+import jiwer
+from normalizers import EnglishTextNormalizer
+
+def get_reference():
+    ref = {}
+    for path in glob.glob('LibriSpeech/*/*/*/*.trans.txt'):
+        with open(path) as fp:
+            for line in fp:
+                code, text = line.strip().split(" ", maxsplit=1)
+                ref [code] = text
+    return ref
+
+def get_hypothesis():
+    hyp = {}
+    for path in glob.glob('LibriSpeech/*/*/*/*.flac.txt'):
+        with open(path) as fp:
+            text = fp.read().strip()
+        code = os.path.basename(path).replace('.flac.txt', '')
+        hyp[code] = text
+    return hyp
+
+def get_codes():
+    codes = []
+    for path in glob.glob('LibriSpeech/*/*/*/*.flac'):
+        codes.append(os.path.basename(path).replace('.flac', ''))
+    return sorted(codes)
+
+def main():
+    normalizer = EnglishTextNormalizer()
+
+    ref_orig = get_reference()
+    hyp_orig = get_hypothesis()
+
+    ref_clean = []
+    hyp_clean = []
+
+    for code in get_codes():
+        ref_clean.append(normalizer(ref_orig[code]))
+        hyp_clean.append(normalizer(hyp_orig[code]))
+
+    wer = jiwer.wer(ref_clean, hyp_clean)
+    print(f"WER: {wer * 100:.2f}%")
+
+if __name__ == '__main__':
+    main()