[TTS] fixed wrong val loss for epoch 0 and inconsistent metrics names
Signed-off-by: Xuesong Yang <1646669+XuesongYang@users.noreply.github.com>

# What does this PR do ?
This PR fixed two issues below,
1. initial checkpoint got wrong `val_loss` so the name was not correct. The root-cause is that `trainer.max_epochs=1` is less than `train.check_val_every_n_epoch=5`. So specify `train.check_val_every_n_epoch=1` would fix the issue for FastPitch. Similar fix for HiFiGAN. See results comparison below,
```
# before fix:
$ ls resultGermanTTS/FastPitch/2022-10-05_12-35-50/checkpoints/
 FastPitch.nemo  'FastPitch--val_loss=0.0000-epoch=1-last.ckpt 
$ ls resultGermanTTS/HifiGan/2022-10-05_19-29-39/checkpoints/
 HifiGan.nemo  'HifiGan--val_loss=0.0000-epoch=1-last.ckpt'

# after fix:
$ ls resultGermanTTS/FastPitch/2022-10-05_14-18-25/checkpoints/
 FastPitch.nemo  'FastPitch--val_loss=4.0512-epoch=0.ckpt'  'FastPitch--val_loss=4.0512-epoch=1-last.ckpt'
$ ls resultGermanTTS/HifiGan/2022-10-05_20-10-50/checkpoints/
 HifiGan.nemo  'HifiGan--val_loss=0.5253-epoch=0.ckpt'  'HifiGan--val_loss=0.5253-epoch=0-last.ckpt'
```
2. TTS loss metrics used `v_loss` rather than `val_loss` as the keyword for validation loss. So this check (https://github.com/NVIDIA/NeMo/blob/r1.12.0/nemo/utils/exp_manager.py#L906) was never succeeded. Rename to `val_loss` helps to make the check valid.

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