Update num worker calculation due to PTL flag changes
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Removes conditionals in favor of simple calculation of num workers due to changes in PTL flags, as per conversation with @nithinraok and prior discussion in #3589 

# Changelog
- Update calculation of num workers in model class (optimizer setup)
- Update calculation of num workers in TTS helpers
- Bump PTL version requirement from 1.6.0 -> 1.6.1

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