feat: define behaviour for empty references

[nikvaessen]

There is undefined behaviour when you apply an empty reference and hypothesis pair
to the WER formula, as you get a division by zero.

As of version 4.0, jiwer defines the behaviour as follows:

import jiwer

# when ref and hyp are both empty, there is no error as
# an ASR system correctly predicted silence/non-speech.
assert jiwer.wer('', '') == 0 
assert jiwer.mer('', '') == 0
assert jiwer.wip('', '') == 1
assert jiwer.wil('', '') == 0

assert jiwer.cer('', '') == 0

When the hypothesis is non-empty, every word or character counts as an insertion:

import jiwer

assert jiwer.wer('', 'silence') == 1
assert jiwer.wer('', 'peaceful silence') == 2
assert jiwer.process_words('', 'now defined behaviour').insertions == 3

assert jiwer.cer('', 'a') == 1
assert jiwer.cer('', 'abcde') == 5

This resolves #98.

[KarelVesely84]

Hello Nik,
what would you think about adding a "NoTransform" placeholder class ?

This would be used, if user wishes to do all the transforms himself, example here:
KarelVesely84@1c5f29d#diff-aae25c8b196777e4abeb01845ca517591f65580977ee7d1f639b5bbf16e26d9d
(plus a registration is necessary in transforms.py header to __all__ variable)

Or can there be something similar already in the code ?

Thank you & best regards
Karel

[nikvaessen]

See #116