preprocess.py misuses Espeak backend, resulting in slow performance and memory leak

[Dachannien]

In ForwardTacotron/utils/text/cleaners.py, the Espeak phonemizer backend is recreated every iteration of the preprocessing, on every text string. Initializing the backend is really slow and appears to pile a bunch of instances of the backend into a dict that something is holding onto somewhere, causing a memory leak. By creating an Espeak phonemizer backend in the Cleaner class and just reusing it for every text string, this saves a huge amount of time and allows the process to complete even on systems with not much memory. See the warning at the phonemizer documentation here: https://bootphon.github.io/phonemizer/common_issues.html

I did not push any fixes myself because (1) I'm a bit of a github noob and (2) I don't know whether the Espeak backend is reentrant, and if it isn't, it's somewhat of a design decision whether multithreading or this fix would be better.

[cschaefer26]

Hi, thanks for pointing it out. I personally don't use espeak anymore, but I will look into it - there could be problems with the multiprocessing imo.

[cschaefer26]

Seems like the fix is easy, here you go: #84