Update TTS Configs from LAMB to AdamW
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Replaces LAMB optimizer (unstable, hangs during multi-GPU training) with AdamW for FastPitch and Mixer-TTS configs.
Second attempt based off of `main` instead of `r1.9.0` (see #4219)

**Collection**: TTS

# Changelog 
- Replace LAMB defaults with AdamW
- Update `fastpitch.py` example script to use existing config

**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation