G2P CTC - allow setting vocab_file from config
jenkins @titu1994 what should i do to move this forward ? thank you. G2P would be @rlangman 
Plus, we cannot write to file inside of training step or val step. File writing must be done offline somewhere else cause ddp will introduce too many cases to it.  file writing already happened here, i didnt change anything about it, except i skip file writing if dict is provided, so i guess for DDP this should work if file is provided and not generated, so i think this should be an improvement for tht case as well jenkins jenkins This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. This PR was closed because it has been inactive for 7 days since being marked as stale. i hope you can reopen this, i was still waiting for the review, as far as i am aware this is ready to merge jenkins jenkins @ppisljar [this test](https://github.com/NVIDIA/NeMo/blob/main/Jenkinsfile#L1216) is still failing  This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. This PR was closed because it has been inactive for 7 days since being marked as stale.

# What does this PR do ?

ctc G2P model always generates the vocab_file which is only suitable for english. This allows you to set custom vocab_file thru config.

**Collection**: TTS/G2p

# Changelog 
- allow passing custom vocab file to grapheme_tokenizer

